zebra: zebra-nht-routemap.patch

zebra: zebra-nht-routemap.patch
Zebra: Add route-map support for Next Hop Tracking

It is sometimes useful to restrict the resolution of recursive routes
to only specific via's. For example, in some configurations resolving
a route through a default route is not acceptable.

This patch adds a new route-map attach point, to zebra's next-hop-tracking
server. Whenever NHT is considering sending notification of a route
resolution, it applies a specified route-map and only if it passes, is the
NHT reachable message sent to the appropriate client protocol (BGP, OSPF etc.).
If the route-map filters the resolution, then a withdraw is sent to the
client protocol.

The route-map is sent the ip address of the route via which the resolution is
happening as well as the valid NHs associated with that route.

We also add support for matching on IP addr prefix len and source protocol
to ensure that resolution happens only via a very specific route.