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 <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/core/skbuff.c

···
 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;
···
 			  unsigned int truesize)
 {
 	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;