ublk: add batch I/O dispatch infrastructure

Add infrastructure for delivering I/O commands to ublk server in batches,
preparing for the upcoming UBLK_U_IO_FETCH_IO_CMDS feature.

Key components:

- struct ublk_batch_fetch_cmd: Represents a batch fetch uring_cmd that will
  receive multiple I/O tags in a single operation, using io_uring's
  multishot command for efficient ublk IO delivery.

- ublk_batch_dispatch(): Batch version of ublk_dispatch_req() that:
  * Pulls multiple request tags from the events FIFO (lock-free reader)
  * Prepares each I/O for delivery (including auto buffer registration)
  * Delivers tags to userspace via single uring_cmd notification
  * Handles partial failures by restoring undelivered tags to FIFO

The batch approach significantly reduces notification overhead by aggregating
multiple I/O completions into single uring_cmd, while maintaining the same
I/O processing semantics as individual operations.

Error handling ensures system consistency: if buffer selection or CQE
posting fails, undelivered tags are restored to the FIFO for retry,
meantime IO state has to be restored.

This runs in task work context, scheduled via io_uring_cmd_complete_in_task()
or called directly from ->uring_cmd(), enabling efficient batch processing
without blocking the I/O submission path.

Reviewed-by: Caleb Sander Mateos <csander@purestorage.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

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