ip6_tun: Add infrastructure for doing encapsulation

Add encap_hlen and ip_tunnel_encap structure to ip6_tnl. Add functions
for getting encap hlen, setting up encap on a tunnel, performing
encapsulation operation.

Signed-off-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>