ublk: enhance check for register/unregister io buffer command

ublk: enhance check for register/unregister io buffer command

The simple check of UBLK_IO_FLAG_OWNED_BY_SRV can avoid incorrect
register/unregister io buffer easily, so check it before calling
starting to register/un-register io buffer.

Also only allow io buffer register/unregister uring_cmd in case of
UBLK_F_SUPPORT_ZERO_COPY.

Also mark argument 'ublk_queue *' of ublk_register_io_buf as const.

Reviewed-by: Caleb Sander Mateos <csander@purestorage.com>
Fixes: 1f6540e2aabb ("ublk: zc register/unregister bvec")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250429022941.1718671-4-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>