syscall: fs_nacl: Unlock() should be deferred.

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