io_uring/zcrx: warn on alloc with non-empty pp cache

Page pool ensures the cache is empty before asking to refill it. Warn if
the assumption is violated.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://patch.msgid.link/9c9792d6e65f3780d57ff83b6334d341ed9a5f29.1774261953.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: isilence

io_uring/zcrx.c

@@ -1075,8 +1075,8 @@ static netmem_ref io_pp_zc_alloc_netmems(struct page_pool *pp, gfp_t gfp)
 	struct io_zcrx_ifq *ifq = io_pp_to_ifq(pp);
 
 	/* pp should already be ensuring that */
-	if (unlikely(pp->alloc.count))
-		goto out_return;
+	if (WARN_ON_ONCE(pp->alloc.count))
+		return 0;
 
 	io_zcrx_ring_refill(pp, ifq);
 	if (likely(pp->alloc.count))