io_uring: make io_do_iopoll return number of reqs

Don't pass nr_events pointer around but return directly, it's less
expensive than pointer increments.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/f771a8153a86f16f12ff4272524e9e549c5de40b.1632516769.git.asml.silence@gmail.com


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