syscall: remove double Unlock from Dup2 on nacl

Fixes #24610

Change-Id: I76dee97db7cd77fe03e4a224f679a5efd061a2b5
Reviewed-on: https://go-review.googlesource.com/103775Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/syscall/fd_nacl.go

@@ -121,7 +121,6 @@ func Dup(fd int) (int, error) {
 
 func Dup2(fd, newfd int) error {
 	files.Lock()
-	defer files.Unlock()
 	if fd < 0 || fd >= len(files.tab) || files.tab[fd] == nil || newfd < 0 || newfd >= len(files.tab)+100 {
 		files.Unlock()
 		return EBADF