ppp: call skb_checksum_complete_unset in ppp_receive_frame

commit 3dfb0534 upstream.

Call checksum_complete_unset in PPP receive to discard checksum-complete
value. PPP does not pull checksum for headers and also modifies packet
as in VJ compression.
Signed-off-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

drivers/net/ppp/ppp_generic.c

@@ -1710,6 +1710,7 @@ ppp_receive_frame(struct ppp *ppp, struct sk_buff *skb, struct channel *pch)
 {
 	/* note: a 0-length skb is used as an error indication */
 	if (skb->len > 0) {
+		skb_checksum_complete_unset(skb);
 #ifdef CONFIG_PPP_MULTILINK
 		/* XXX do channel-level decompression here */
 		if (PPP_PROTO(skb) == PPP_MP)