ipv6/ping: set skb->mark on icmpv6 sockets

IPv6 ping sockets route based on fwmark, but do not yet set skb->mark.
Add this. IPv4 ping sockets also do both.

Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>