io_uring: shuffle rarely used ctx fields

There is a bunch of scattered around ctx fields that are almost never
used, e.g. only on ring exit, plunge them to the end, better locality,
better aesthetically.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/782ff94b00355923eae757d58b1a47821b5b46d4.1621201931.git.asml.silence@gmail.com


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