go.net/ipv6: drop unnecessary constant conversions

Update golang/go#5538

R=dave
CC=golang-dev
https://golang.org/cl/9834046

ipv6/sockopt_rfc3493_windows.go

@@ -24,7 +24,7 @@ func setIPv6TrafficClass(fd syscall.Handle, v int) error {
 func ipv6HopLimit(fd syscall.Handle) (int, error) {
 	var v int32
 	l := int32(4)
-	if err := syscall.Getsockopt(fd, int32(ianaProtocolIPv6), int32(syscall.IPV6_UNICAST_HOPS), (*byte)(unsafe.Pointer(&v)), &l); err != nil {
+	if err := syscall.Getsockopt(fd, ianaProtocolIPv6, syscall.IPV6_UNICAST_HOPS, (*byte)(unsafe.Pointer(&v)), &l); err != nil {
 		return 0, os.NewSyscallError("getsockopt", err)
 	}
 	return int(v), nil
@@ -32,7 +32,7 @@ func ipv6HopLimit(fd syscall.Handle) (int, error) {
 
 func setIPv6HopLimit(fd syscall.Handle, v int) error {
 	vv := int32(v)
-	return os.NewSyscallError("setsockopt", syscall.Setsockopt(fd, int32(ianaProtocolIPv6), int32(syscall.IPV6_UNICAST_HOPS), (*byte)(unsafe.Pointer(&vv)), 4))
+	return os.NewSyscallError("setsockopt", syscall.Setsockopt(fd, ianaProtocolIPv6, syscall.IPV6_UNICAST_HOPS, (*byte)(unsafe.Pointer(&vv)), 4))
 }
 
 func ipv6Checksum(fd syscall.Handle) (bool, int, error) {
@@ -43,7 +43,7 @@ func ipv6Checksum(fd syscall.Handle) (bool, int, error) {
 func ipv6MulticastHopLimit(fd syscall.Handle) (int, error) {
 	var v int32
 	l := int32(4)
-	if err := syscall.Getsockopt(fd, int32(ianaProtocolIPv6), int32(syscall.IPV6_MULTICAST_HOPS), (*byte)(unsafe.Pointer(&v)), &l); err != nil {
+	if err := syscall.Getsockopt(fd, ianaProtocolIPv6, syscall.IPV6_MULTICAST_HOPS, (*byte)(unsafe.Pointer(&v)), &l); err != nil {
 		return 0, os.NewSyscallError("getsockopt", err)
 	}
 	return int(v), nil
@@ -51,13 +51,13 @@ func ipv6MulticastHopLimit(fd syscall.Handle) (int, error) {
 
 func setIPv6MulticastHopLimit(fd syscall.Handle, v int) error {
 	vv := int32(v)
-	return os.NewSyscallError("setsockopt", syscall.Setsockopt(fd, int32(ianaProtocolIPv6), int32(syscall.IPV6_MULTICAST_HOPS), (*byte)(unsafe.Pointer(&vv)), 4))
+	return os.NewSyscallError("setsockopt", syscall.Setsockopt(fd, ianaProtocolIPv6, syscall.IPV6_MULTICAST_HOPS, (*byte)(unsafe.Pointer(&vv)), 4))
 }
 
 func ipv6MulticastInterface(fd syscall.Handle) (*net.Interface, error) {
 	var v int32
 	l := int32(4)
-	if err := syscall.Getsockopt(fd, int32(ianaProtocolIPv6), int32(syscall.IPV6_MULTICAST_IF), (*byte)(unsafe.Pointer(&v)), &l); err != nil {
+	if err := syscall.Getsockopt(fd, ianaProtocolIPv6, syscall.IPV6_MULTICAST_IF, (*byte)(unsafe.Pointer(&v)), &l); err != nil {
 		return nil, os.NewSyscallError("getsockopt", err)
 	}
 	if v == 0 {
@@ -75,13 +75,13 @@ func setIPv6MulticastInterface(fd syscall.Handle, ifi *net.Interface) error {
 	if ifi != nil {
 		v = int32(ifi.Index)
 	}
-	return os.NewSyscallError("setsockopt", syscall.Setsockopt(fd, int32(ianaProtocolIPv6), int32(syscall.IPV6_MULTICAST_IF), (*byte)(unsafe.Pointer(&v)), 4))
+	return os.NewSyscallError("setsockopt", syscall.Setsockopt(fd, ianaProtocolIPv6, syscall.IPV6_MULTICAST_IF, (*byte)(unsafe.Pointer(&v)), 4))
 }
 
 func ipv6MulticastLoopback(fd syscall.Handle) (bool, error) {
 	var v int32
 	l := int32(4)
-	if err := syscall.Getsockopt(fd, int32(ianaProtocolIPv6), int32(syscall.IPV6_MULTICAST_LOOP), (*byte)(unsafe.Pointer(&v)), &l); err != nil {
+	if err := syscall.Getsockopt(fd, ianaProtocolIPv6, syscall.IPV6_MULTICAST_LOOP, (*byte)(unsafe.Pointer(&v)), &l); err != nil {
 		return false, os.NewSyscallError("getsockopt", err)
 	}
 	return v == 1, nil
@@ -89,7 +89,7 @@ func ipv6MulticastLoopback(fd syscall.Handle) (bool, error) {
 
 func setIPv6MulticastLoopback(fd syscall.Handle, v bool) error {
 	vv := int32(boolint(v))
-	return os.NewSyscallError("setsockopt", syscall.Setsockopt(fd, int32(ianaProtocolIPv6), int32(syscall.IPV6_MULTICAST_LOOP), (*byte)(unsafe.Pointer(&vv)), 4))
+	return os.NewSyscallError("setsockopt", syscall.Setsockopt(fd, ianaProtocolIPv6, syscall.IPV6_MULTICAST_LOOP, (*byte)(unsafe.Pointer(&vv)), 4))
 }
 
 func joinIPv6Group(fd syscall.Handle, ifi *net.Interface, grp net.IP) error {
@@ -98,7 +98,7 @@ func joinIPv6Group(fd syscall.Handle, ifi *net.Interface, grp net.IP) error {
 	if ifi != nil {
 		mreq.Interface = uint32(ifi.Index)
 	}
-	return os.NewSyscallError("setsockopt", syscall.Setsockopt(fd, int32(ianaProtocolIPv6), int32(syscall.IPV6_JOIN_GROUP), (*byte)(unsafe.Pointer(&mreq)), int32(unsafe.Sizeof(mreq))))
+	return os.NewSyscallError("setsockopt", syscall.Setsockopt(fd, ianaProtocolIPv6, syscall.IPV6_JOIN_GROUP, (*byte)(unsafe.Pointer(&mreq)), int32(unsafe.Sizeof(mreq))))
 }
 
 func leaveIPv6Group(fd syscall.Handle, ifi *net.Interface, grp net.IP) error {
@@ -107,7 +107,7 @@ func leaveIPv6Group(fd syscall.Handle, ifi *net.Interface, grp net.IP) error {
 	if ifi != nil {
 		mreq.Interface = uint32(ifi.Index)
 	}
-	return os.NewSyscallError("setsockopt", syscall.Setsockopt(fd, int32(ianaProtocolIPv6), int32(syscall.IPV6_LEAVE_GROUP), (*byte)(unsafe.Pointer(&mreq)), int32(unsafe.Sizeof(mreq))))
+	return os.NewSyscallError("setsockopt", syscall.Setsockopt(fd, ianaProtocolIPv6, syscall.IPV6_LEAVE_GROUP, (*byte)(unsafe.Pointer(&mreq)), int32(unsafe.Sizeof(mreq))))
 }
 
 func setIPv6Checksum(fd syscall.Handle, on bool, offset int) error {