[IPV6] IP6TUNNEL: Enable to control the handled inner protocol.

ip6_tunnel before supporting IPv4/IPv6 tunnel allows only IPPROTO_IPV6
in configurations from userland. This allows userland to set IPPROTO_IPIP
and 0(wildcard). ip6_tunnel only handles allowed inner protocols.

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>