dm ioctl: log an error if the ioctl structure is corrupted

This will help triage bugs when userspace is passing invalid ioctl
structure to the kernel.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
[snitzer: log errors using DMERR instead of DMWARN]
Signed-off-by: Mike Snitzer <snitzer@kernel.org>