ipv6: sr: implement additional seg6local actions

This patch implements the following seg6local actions.

- SEG6_LOCAL_ACTION_END_T: regular SRH processing and forward to the
  next-hop looked up in the specified routing table.

- SEG6_LOCAL_ACTION_END_DX2: decapsulate an L2 frame and forward it to
  the specified network interface.

- SEG6_LOCAL_ACTION_END_DX4: decapsulate an IPv4 packet and forward it,
  possibly to the specified next-hop.

- SEG6_LOCAL_ACTION_END_DT6: decapsulate an IPv6 packet and forward it
  to the next-hop looked up in the specified routing table.

Signed-off-by: David Lebrun <david.lebrun@uclouvain.be>
Signed-off-by: David S. Miller <davem@davemloft.net>