io_uring: add helper io_uring_cmd_ctx_handle()

Add helper io_uring_cmd_ctx_handle() for driver to track per-context
resource, such as registered kernel io buffer.

Suggested-by: Caleb Sander Mateos <csander@purestorage.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Caleb Sander Mateos <csander@purestorage.com>
Link: https://lore.kernel.org/r/20250522152043.399824-2-ming.lei@redhat.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>