io_uring: simplify io_prep_linked_timeout

The link test in io_prep_linked_timeout() is pretty bulky, replace it
with a flag. It's better for normal path and linked requests, and also
will be used further for request failing.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/3703770bfae8bc1ff370e43ef5767940202cab42.1628981736.git.asml.silence@gmail.com


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