ppp: fix truesize underestimation

When building frag_list, head truesize should be sum of all frag
truesize.

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

drivers/net/ppp/ppp_generic.c

···
 
 				skb->len += p->len;
 				skb->data_len += p->len;
-				skb->truesize += p->len;
+				skb->truesize += p->truesize;
 
 				if (p == tail)
 					break;