io_uring: de-unionise io_kiocb

As io_kiocb have enough space, move ->work out of a union. It's safer
this way and removes ->work memcpy bouncing.
By the way make tabulation in struct io_kiocb consistent.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>