io_uring: IRQ rw completion batching

Employ inline completion logic for read/write completions done via
io_req_task_complete(). If ->uring_lock is contended, just do normal
request completion, but if not, make tctx_task_work() to grab the lock
and do batched inline completions in io_req_task_complete().

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/94589c3ce69eaed86a21bb1ec696407a54fab1aa.1629286357.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>