io_uring: small io_submit_sqe() optimisation

submit_state.link is used only to assemble a link and not used for
actual submission, so clear it before io_queue_sqe() in io_submit_sqe(),
awhile it's hot and in caches and queueing doesn't spoil it. May also
potentially help compiler with spilling or to do other optimisations.
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/1579939426f3ad6b55af3005b1389bbbed7d780d.1623709150.git.asml.silence@gmail.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -6616,8 +6616,8 @@ static int io_submit_sqe(struct io_ring_ctx *ctx, struct io_kiocb *req,
 
 		/* last request of a link, enqueue the link */
 		if (!(req->flags & (REQ_F_LINK | REQ_F_HARDLINK))) {
-			io_queue_sqe(head);
 			link->head = NULL;
+			io_queue_sqe(head);
 		}
 	} else {
 		if (unlikely(ctx->drain_next)) {