ppp: fix truesize underestimation

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

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c
index edfa15d..93a8639 100644
--- a/drivers/net/ppp/ppp_generic.c
+++ b/drivers/net/ppp/ppp_generic.c
@@ -2113,7 +2113,7 @@
 
 				skb->len += p->len;
 				skb->data_len += p->len;
-				skb->truesize += p->len;
+				skb->truesize += p->truesize;
 
 				if (p == tail)
 					break;