autofs: fix slab out of bounds read in getname_kernel()

The autofs subsystem does not check that the "path" parameter is present
for all cases where it is required when it is passed in via the "param"
struct.

In particular it isn't checked for the AUTOFS_DEV_IOCTL_OPENMOUNT_CMD
ioctl command.

To solve it, modify validate_dev_ioctl(function to check that a path has
been provided for ioctl commands that require it.

Link: http://lkml.kernel.org/r/153060031527.26631.18306637892746301555.stgit@pluto.themaw.net


Signed-off-by: Tomas Bortoli <tomasbortoli@gmail.com>
Signed-off-by: Ian Kent <raven@themaw.net>
Reported-by:  <syzbot+60c837b428dc84e83a93@syzkaller.appspotmail.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>