io_uring: uring_cmd: add multishot support

Add UAPI flag IORING_URING_CMD_MULTISHOT for supporting multishot
uring_cmd operations with provided buffer.

This enables drivers to post multiple completion events from a single
uring_cmd submission, which is useful for:

- Notifying userspace of device events (e.g., interrupt handling)
- Supporting devices with multiple event sources (e.g., multi-queue devices)
- Avoiding the need for device poll() support when events originate
  from multiple sources device-wide

The implementation adds two new APIs:
- io_uring_cmd_select_buffer(): selects a buffer from the provided
  buffer group for multishot uring_cmd
- io_uring_mshot_cmd_post_cqe(): posts a CQE after event data is
  pushed to the provided buffer

Multishot uring_cmd must be used with buffer select (IOSQE_BUFFER_SELECT)
and is mutually exclusive with IORING_URING_CMD_FIXED for now.

The ublk driver will be the first user of this functionality:

	https://github.com/ming1/linux/commits/ublk-devel/



Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250821040210.1152145-3-ming.lei@redhat.com


[axboe: fold in fix for !CONFIG_IO_URING]
Signed-off-by: Jens Axboe <axboe@kernel.dk>