syscall: fs_nacl: Unlock() should be deferred.

Change-Id: Iea08b8f4e74a8cd4b4d317273046457c8db956a1
Reviewed-on: https://go-review.googlesource.com/1640Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/syscall/fs_nacl.go

@@ -815,7 +815,7 @@ func fdToFsysFile(fd int) (*fsysFile, error) {
 // It is meant to be called when initializing the file system image.
 func create(name string, mode uint32, sec int64, data []byte) error {
 	fs.mu.Lock()
-	fs.mu.Unlock()
+	defer fs.mu.Unlock()
 	f, err := fs.open(name, O_CREATE|O_EXCL, mode)
 	if err != nil {
 		if mode&S_IFMT == S_IFDIR {