dm-ioctl: report an error if a device has no table

When we send a message to a device that has no table, the return code was
not set. The code would return "2", which is not considered a valid return value.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Cc: stable@vger.kernel.org
Reviewed-by: Benjamin Marzinski <bmarzins@redhat.com>