io_uring: make timeout prep handlers consistent with other prep handlers

All other opcodes take a {req, sqe} set for prep handling, split out
a timeout prep handler so that timeout and linked timeouts can use
the same one.

Reviewed-by: Kanchan Joshi <joshi.k@samsung.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>