io_uring: remove iopoll spinlock

This reverts commit 2ccc92f4

io_req_complete_post() should now behave well even in case of IOPOLL, we
can remove completion_lock locking.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/7e171c8b530656b14a671c59100ca260e46e7f2a.1669203009.git.asml.silence@gmail.com


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