rtnetlink: introduce new RTA_ENCAP_TYPE and RTA_ENCAP attributes

This patch introduces two new RTA attributes to attach encap
data to fib routes.

Example iproute2 command to attach mpls encap data to ipv4 routes

$ip route add 10.1.1.0/30 encap mpls 200 via inet 10.1.1.1 dev swp1

Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Suggested-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>