autofs: use autofs4_free_ino() to kfree dentry data

kfree dentry data allocated by autofs4_new_ino() with autofs4_free_ino()
instead of raw kfree.  (since we have the interface to free autofs_info*)

This patch was modified to remove the need to set the dentry info field to
NULL dew to a change in the previous patch.

Link: http://lkml.kernel.org/r/20160812024805.12352.43650.stgit@pluto.themaw.net


Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@gmail.com>
Signed-off-by: Ian Kent <raven@themaw.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>