unix: add missing dirent* helper functions on aix

These are needed by ParseDirent and were missing in CL 183897.

Change-Id: I5b340fea9c0dc1b65b717b0d3cfd8cbb40d3cae9
Reviewed-on: https://go-review.googlesource.com/c/sys/+/183997
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall_aix.go

@@ -280,6 +280,22 @@ func sendfile(outfd int, infd int, offset *int64, count int) (written int, err e
 	return -1, ENOSYS
 }
 
+func direntIno(buf []byte) (uint64, bool) {
+	return readInt(buf, unsafe.Offsetof(Dirent{}.Ino), unsafe.Sizeof(Dirent{}.Ino))
+}
+
+func direntReclen(buf []byte) (uint64, bool) {
+	return readInt(buf, unsafe.Offsetof(Dirent{}.Reclen), unsafe.Sizeof(Dirent{}.Reclen))
+}
+
+func direntNamlen(buf []byte) (uint64, bool) {
+	reclen, ok := direntReclen(buf)
+	if !ok {
+		return 0, false
+	}
+	return reclen - uint64(unsafe.Offsetof(Dirent{}.Name)), true
+}
+
 //sys	getdirent(fd int, buf []byte) (n int, err error)
 func Getdents(fd int, buf []byte) (n int, err error) {
 	return getdirent(fd, buf)