io_uring: refill request cache in memory order

The allocator will generally return memory in order, but
__io_alloc_req_refill() then adds them to a stack and we'll extract them
in the opposite order. This obviously isn't a huge deal, but:

1) it makes debugging easier when they are in order
2) keeping them in-order is the right thing to do
3) reduces the code for adding them to the stack

Just add them in reverse to the stack.
Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/io_uring.c

@@ -1039,7 +1039,7 @@ __cold bool __io_alloc_req_refill(struct io_ring_ctx *ctx)
 {
 	gfp_t gfp = GFP_KERNEL | __GFP_NOWARN;
 	void *reqs[IO_REQ_ALLOC_BATCH];
-	int ret, i;
+	int ret;
 
 	/*
 	 * If we have more than a batch's worth of requests in our IRQ side
@@ -1066,8 +1066,8 @@ __cold bool __io_alloc_req_refill(struct io_ring_ctx *ctx)
 	}
 
 	percpu_ref_get_many(&ctx->refs, ret);
-	for (i = 0; i < ret; i++) {
-		struct io_kiocb *req = reqs[i];
+	while (ret--) {
+		struct io_kiocb *req = reqs[ret];
 
 		io_preinit_req(req, ctx);
 		io_req_add_to_cache(req, ctx);