fs: don't randomize struct kiocb fields

This is a size sensitive structure and randomizing can introduce extra
padding that breaks io_uring's fixed size expectations. There are few
fields here as it is, half of which need a fixed order to optimally
pack, so the randomization isn't providing much.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Keith Busch <kbusch@kernel.org>
Link: https://lore.kernel.org/io-uring/b6f508ca-b1b2-5f40-7998-e4cff1cf7212@kernel.dk/


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