io_uring: comment why inline complete calls io_clean_op()

io_req_complete_state() calls io_clean_op() and it may be not entirely
obvious, leave a comment.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/21806f862151e223fdf439e5e8ed7178a8d66979.1632516769.git.asml.silence@gmail.com


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