autofs4 - fix deal with autofs4_write races

I don't know how I missed this obvious mistake when I
reviewed Als' patches, sorry.

[ Quoting Al:

	Grr...  Note to self: do git status *and* git stash show -p
	before git push.  Nothing like "WTF? I'd fixed that braino"
	feeling ;-/

  Al sent the same patch - it got broken in commit d668dc56:
  "autofs4: deal with autofs4_write/autofs4_write races". ]
Reported-and-tested-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Ian Kent <raven@themaw.net>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/autofs4/waitq.c

@@ -76,7 +76,7 @@ static int autofs4_write(struct autofs_sb_info *sbi,
 		data += wr;
 		bytes -= wr;
 	}
-	mutex_lock(&sbi->pipe_mutex);
+	mutex_unlock(&sbi->pipe_mutex);
 
 	set_fs(fs);