dm ioctl: fix a couple ioctl codes

Change the ioctl codes from DM_DEV_ARM_POLL to DM_DEV_ARM_POLL_CMD and
from DM_GET_TARGET_VERSION to DM_GET_TARGET_VERSION_CMD.

Note that the "cmd" field of "struct _ioctls" is never used, thus this
commit doesn't fix any bug, it just makes the code consistent.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>