ppp: fix truesize underestimation

When building frag_list, head truesize should be sum of all frag
truesize.
Signed-off-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ppp/ppp_generic.c

@@ -2113,7 +2113,7 @@ ppp_mp_reconstruct(struct ppp *ppp)
 
 				skb->len += p->len;
 				skb->data_len += p->len;
-				skb->truesize += p->len;
+				skb->truesize += p->truesize;
 
 				if (p == tail)
 					break;