[IPV6] IP6TUNNEL: Split out generic routine in ip6ip6_rcv().

This enables to add IPv4/IPv6 specific handling later,

Signed-off-by: Yasuyuki Kozakai <yasuyuki.kozakai@toshiba.co.jp>
Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>