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: NRoopa Prabhu <roopa@cumulusnetworks.com>
Suggested-by: NEric W. Biederman <ebiederm@xmission.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/uapi/linux/rtnetlink.h

@@ -308,6 +308,8 @@ enum rtattr_type_t {
 	RTA_VIA,
 	RTA_NEWDST,
 	RTA_PREF,
+	RTA_ENCAP_TYPE,
+	RTA_ENCAP,
 	__RTA_MAX
 };