io_uring: decouple CQE filling from requests

Make __io_cqring_fill_event() agnostic of struct io_kiocb, pass all the
data needed directly into it. Will be used to post rsrc removal
completions, which don't have an associated request.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/c9b8da9e42772db2033547dfebe479dc972a0f2c.1619356238.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>