bsg: add io_uring command support to generic layer

bsg: add io_uring command support to generic layer

Add an io_uring command handler to the generic BSG layer. The new
.uring_cmd file operation validates io_uring features and delegates
handling to a per-queue bsg_uring_cmd_fn callback.

Extend bsg_register_queue() so transport drivers can register both
sg_io and io_uring command handlers.

Signed-off-by: Yang Xiuwei <yangxiuwei@kylinos.cn>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Link: https://patch.msgid.link/20260317072226.2598233-3-yangxiuwei@kylinos.cn
Signed-off-by: Jens Axboe <axboe@kernel.dk>