block/null_blk: add queue_rqs() support

Add batched mq_ops.queue_rqs() support in null_blk for testing. The
implementation is much easy since null_blk doesn't have commit_rqs().

We simply handle each request one by one, if errors are encountered,
leave them in the passed in list and return back.

There is about 3.6% improvement in IOPS of fio/t/io_uring on null_blk
with hw_queue_depth=256 on my test VM, from 1.09M to 1.13M.

Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20230913151616.3164338-6-chengming.zhou@linux.dev


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