ipv6: ping: fix wrong checksum for large frames

For a given ping datagram, ping_getfrag() is called once
per skb fragment.

A large datagram requiring more than one page fragment
is currently getting the checksum of the last fragment,
instead of the cumulative one.

After this patch, "ping -s 35000 ::1" is working correctly.

Fixes: 6d0bfe2 ("net: ipv6: Add IPv6 support to the ping socket.")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Lorenzo Colitti <lorenzo@google.com>
Cc: Maciej Żenczykowski <maze@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ping.c

@@ -639,7 +639,7 @@ int ping_getfrag(void *from, char *to,
 	 * wcheck, it will be finalized in ping_v4_push_pending_frames.
 	 */
 	if (pfh->family == AF_INET6) {
-		skb->csum = pfh->wcheck;
+		skb->csum = csum_block_add(skb->csum, pfh->wcheck, odd);
 		skb->ip_summed = CHECKSUM_NONE;
 		pfh->wcheck = 0;
 	}