io_uring: helper for prep+queuing linked timeouts

We try to aggresively inline the submission path, so it's a good idea to
not pollute it with colder code. One of them is linked timeout
preparation + queue, which can be extracted into a function.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/ecf74df7ac77389b6d9211211ec4954e91de98ba.1650056133.git.asml.silence@gmail.com


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