unix: fix cpuset size argument in sched_affinity syscall

Fixes golang/go#23639

Change-Id: I7793dc50de35791a7c8af1966bb2ee4a36a2ac1c
Reviewed-on: https://go-review.googlesource.com/91375
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

unix/affinity_linux.go

@@ -16,7 +16,7 @@ const cpuSetSize = _CPU_SETSIZE / _NCPUBITS
 type CPUSet [cpuSetSize]cpuMask
 
 func schedAffinity(trap uintptr, pid int, set *CPUSet) error {
-	_, _, e := RawSyscall(trap, uintptr(pid), uintptr(unsafe.Sizeof(set)), uintptr(unsafe.Pointer(set)))
+	_, _, e := RawSyscall(trap, uintptr(pid), uintptr(unsafe.Sizeof(*set)), uintptr(unsafe.Pointer(set)))
 	if e != 0 {
 		return errnoErr(e)
 	}