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: 6d0bfe22 ("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>