autofs: path_{get,put}() cleanups

Here are some more places where path_{get,put}() can be used instead of
dput()/mntput() pair.  Besides that it fixes a bug in autofs4_mount_busy()
where mntput() was called before dput().

Signed-off-by: Jan Blunck <jblunck@suse.de>
Cc: Ian Kent <raven@themaw.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>