ip_tunnels: record IP version in tunnel info

There's currently nothing preventing directing packets with IPv6
encapsulation data to IPv4 tunnels (and vice versa). If this happens,
IPv6 addresses are incorrectly interpreted as IPv4 ones.

Track whether the given ip_tunnel_key contains IPv4 or IPv6 data. Store this
in ip_tunnel_info. Reject packets at appropriate places if they are supposed
to be encapsulated into an incompatible protocol.

Signed-off-by: Jiri Benc <jbenc@redhat.com>
Acked-by: Alexei Starovoitov <ast@plumgrid.com>
Acked-by: Thomas Graf <tgraf@suug.ch>
Acked-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>