io_uring: optimise initial ltimeout refcounting

Linked timeouts are never refcounted when it comes to the first call to
__io_prep_linked_timeout(), so save an io_ref_get() and set the desired
value directly.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/177b24cc62ffbb42d915d6eb9e8876266e4c0d5a.1628981736.git.asml.silence@gmail.com


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