io_uring: simplify struct io_uring_sqe layout

Flatten struct io_uring_sqe, the last union is exactly 64B, so move them
out of union { struct { ... }}, and decrease __pad2 size.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/2e21ef7aed136293d654450bc3088973a8adc730.1624543113.git.asml.silence@gmail.com


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