sit: update frag_off info

After parsing the sit netlink change info, we forget to update frag_off in
ipip6_tunnel_update(). Fix it by assigning frag_off with new value.

Reported-by: Jianlin Shi <jishi@redhat.com>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Acked-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>