unix: fix missing use of use function in Getfsstat and Getwd

Updates golang/go#13372.

Change-Id: I623de97eb19880356148cbcb7d17759df82684aa
Reviewed-on: https://go-review.googlesource.com/24751
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall_openbsd.go

@@ -111,6 +111,7 @@ func Getfsstat(buf []Statfs_t, flags int) (n int, err error) {
 		bufsize = unsafe.Sizeof(Statfs_t{}) * uintptr(len(buf))
 	}
 	r0, _, e1 := Syscall(SYS_GETFSSTAT, uintptr(_p0), bufsize, uintptr(flags))
+	use(unsafe.Pointer(_p0))
 	n = int(r0)
 	if e1 != 0 {
 		err = e1