io_uring: introduce io_req_cqe_overflow()

__io_fill_cqe_req() is hot and inlined, we want it to be as small as
possible. Add io_req_cqe_overflow() accepting only a request and doing
all overflow accounting, and replace with it two calls to 6 argument
io_cqring_event_overflow().

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/048b9fbcce56814d77a1a540409c98c3d383edcb.1655455613.git.asml.silence@gmail.com


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