autofs: fix autofs_sbi() does not check super block type

commit 0633da48f0793aeba27f82d30605624416723a91 upstream.

autofs_sbi() does not check the superblock magic number to verify it has
been given an autofs super block.

Backport Note: autofs4 has been renamed to autofs upstream. As a result
the upstream patch does not apply cleanly onto 4.14.y.

Link: http://lkml.kernel.org/r/153475422934.17131.7563724552005298277.stgit@pluto.themaw.net


Reported-by:  <syzbot+87c3c541582e56943277@syzkaller.appspotmail.com>
Signed-off-by: Ian Kent <raven@themaw.net>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Zubin Mithra <zsm@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>