io_uring: silence io_for_each_link() warning

Some tooling keep complaining about self assignment in
io_for_each_link(), the code is correct but still let's workaround it.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/f0de77b0b0f8309554ba6fba34327b7813bcc3ff.1648209006.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -6959,9 +6959,10 @@ static int io_req_prep_async(struct io_kiocb *req)
 static u32 io_get_sequence(struct io_kiocb *req)
 {
 	u32 seq = req->ctx->cached_sq_head;
+	struct io_kiocb *cur;
 
 	/* need original cached_sq_head, but it was increased for each req */
-	io_for_each_link(req, req)
+	io_for_each_link(cur, req)
 		seq--;
 	return seq;
 }