net: add truesize debug checks in skb_{add|coalesce}_rx_frag()

It can be time consuming to track driver bugs, that might be detected
too late from this confusing warning in skb_try_coalesce()

	WARN_ON_ONCE(delta < len);

Add sanity check in skb_add_rx_frag() and skb_coalesce_rx_frag()
to better track bug origin for CONFIG_DEBUG_NET=y builds.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/skbuff.c

@@ -847,6 +847,8 @@ EXPORT_SYMBOL(__napi_alloc_skb);
 void skb_add_rx_frag(struct sk_buff *skb, int i, struct page *page, int off,
 		     int size, unsigned int truesize)
 {
+	DEBUG_NET_WARN_ON_ONCE(size > truesize);
+
 	skb_fill_page_desc(skb, i, page, off, size);
 	skb->len += size;
 	skb->data_len += size;
@@ -859,6 +861,8 @@ void skb_coalesce_rx_frag(struct sk_buff *skb, int i, int size,
 {
 	skb_frag_t *frag = &skb_shinfo(skb)->frags[i];
 
+	DEBUG_NET_WARN_ON_ONCE(size > truesize);
+
 	skb_frag_size_add(frag, size);
 	skb->len += size;
 	skb->data_len += size;