ublk: add io events fifo structure

ublk: add io events fifo structure

Add ublk io events fifo structure and prepare for supporting command
batch, which will use io_uring multishot uring_cmd for fetching one
batch of io commands each time.

One nice feature of kfifo is to allow multiple producer vs single
consumer. We just need lock the producer side, meantime the single
consumer can be lockless.

The producer is actually from ublk_queue_rq() or ublk_queue_rqs(), so
lock contention can be eased by setting proper blk-mq nr_queues.

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>