ipv6: fix 4in6 tunnel receive path

Protocol for 4in6 tunnel is IPPROTO_IPIP. This was wrongly changed by
the last cleanup.

CC: Tom Herbert <tom@herbertland.com>
Fixes: 0d3c703a

 ("ipv6: Cleanup IPv6 tunnel receive path")
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>