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: NPavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/177b24cc62ffbb42d915d6eb9e8876266e4c0d5a.1628981736.git.asml.silence@gmail.comSigned-off-by: NJens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -1312,8 +1312,7 @@ static struct io_kiocb *__io_prep_linked_timeout(struct io_kiocb *req)
 
 	/* linked timeouts should have two refs once prep'ed */
 	io_req_set_refcount(req);
-	io_req_set_refcount(nxt);
-	req_ref_get(nxt);
+	__io_req_set_refcount(nxt, 2);
 
 	nxt->timeout.head = req;
 	nxt->flags |= REQ_F_LTIMEOUT_ACTIVE;