net: fix pointer check in skb_pp_cow_data routine

Properly check page pointer returned by page_pool_dev_alloc routine in
skb_pp_cow_data() for non-linear part of the original skb.
Reported-by: Julian Wiedmann <jwiedmann.dev@gmail.com>
Closes: https://lore.kernel.org/netdev/cover.1707729884.git.lorenzo@kernel.org/T/#m7d189b0015a7281ed9221903902490c03ed19a7a
Fixes: e6d5dbdd ("xdp: add multi-buff support for xdp running in generic mode")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Link: https://lore.kernel.org/r/25512af3e09befa9dcb2cf3632bdc45b807cf330.1708167716.git.lorenzo@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/skbuff.c

@@ -968,7 +968,7 @@ int skb_pp_cow_data(struct page_pool *pool, struct sk_buff **pskb,
 		truesize = size;
 
 		page = page_pool_dev_alloc(pool, &page_off, &truesize);
-		if (!data) {
+		if (!page) {
 			consume_skb(nskb);
 			return -ENOMEM;
 		}