ipv6: ioam: Add support for the ip6ip6 encapsulation

This patch adds support for the ip6ip6 encapsulation by providing three encap
modes: inline, encap and auto.

Signed-off-by: Justin Iurman <justin.iurman@uliege.be>
Signed-off-by: David S. Miller <davem@davemloft.net>