mpls: ip tunnel support

This implementation uses lwtunnel infrastructure to register
hooks for mpls tunnel encaps.

It picks cues from iptunnel_encaps infrastructure and previous
mpls iptunnel RFC patches from Eric W. Biederman and Robert Shearman

Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>