ublk: fix UBLK_CMD_DEL_DEV_ASYNC handling

[ Upstream commit 55fbb9a5d64e0e590cad5eacc16c99f2482a008f ]

In ublk_ctrl_uring_cmd(), ioctl command NR should be used for
matching _IOC_NR(cmd_op).

Fix it by adding one private macro, and this way is clean.

Fixes: 13fe8e68

 ("ublk: add UBLK_CMD_DEL_DEV_ASYNC")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20240724143311.2646330-1-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>