Add support for the SRv6 End.DT46 behavior

[cscarpitta]

This PR adds the support for the SRv6 End.DT46 behavior.

The SRv6 End.DT46 behavior is used to implement L3VPN use-cases in multi-tenants environments. It decapsulates the received packets and it performs IPv4 or IPv6 routing lookup in the routing table of the tenant.

The SRv6 End.DT46 behavior has been introduced in the Linux kernel in the commit [1] and it is available since kernel version 5.15.
The iproute2 suite has been extended as well for supporting the SRv6 End.DT46 behavior [2].

An example iproute2 command to instantiate an SRv6 End.DT46 behavior is the following:

$ ip -6 route add 2001:db8::1 encap seg6local action End.DT46 vrftable 100 dev vrf100.

In the above command, the vrftable attribute indicates the routing table associated with the VRF device used by SRv6 End.DT46 for routing IP packets.

The only prerequisite to instantiate an End.DT46 behavior is enabling the VRF strict_mode sysctl parameter

$ sysctl -wq net.vrf.strict_mode=1

Hereafter, we provide an example of how to instantiate an SRv6 End.DT46 behavior with pyroute2.

First, a VRF needs to be created:

	ip.link('add',
	           ifname='vrf-foo',
	           kind='vrf',
	           vrf_table=100)

Then, the following command can be used to instantiate the behavior:

	ip.route('add',
	             dst='2001:db8::1/128',
	             oif=idx,
	             encap={'type': 'seg6local',
	                          action': 'End.DT46',
	                          'vrf_table': 100})

[1] https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git/commit/?id=8b532109bf885b7b59b93487bc4672eb6d071b78
[2] https://www.spinics.net/lists/netdev/msg749223.html

[svinota]

Wow. Great, thanks a lot!