io_uring: refactor ctx slow data placement

Shove all slow path data at the end of ctx and get rid of extra
indention.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/bcaf200298dd469af20787650550efc66d89bef2.1655310733.git.asml.silence@gmail.com


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