afs: mntput called before dput

dput must be called before mntput here.

Signed-off-by: Andreas Gruenbacher <agruen@suse.de>
Acked-By: David Howells <dhowells@redhat.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>