zebra: Nexthop tracking, route resolution recursive lookup

zebra: Nexthop tracking, route resolution recursive lookup

Description:
===========
Change is intended for fixing the NHT resolution logic.
While recursively resolving nexthop, keep looking for a valid/useable route in the rib,
by not stopping at the first/most-specific route in the rib.

Consider the following set of events taking place on R1:
R1(config)# ip route 2.2.2.0/24 ens192
R1# sharp watch nexthop 2.2.2.32 connected
R1# show ip nht
2.2.2.32(Connected)
 resolved via static
 is directly connected, ens192
 Client list: sharp(fd 33)

-2.2.2.32 NHT is resolved over the above valid static route.

R1# sharp install routes 2.2.2.32 nexthop 2.2.2.32 1
R1# 2.2.2.32(Connected)
 resolved via static
 is directly connected, ens192
 Client list: sharp(fd 33)

-.32/32 comes which is going to resolve through itself, but since this is an invalid route,
it will be marked as inactive and will not affect the NHT.

R1# sharp install routes 2.2.2.31 nexthop 2.2.2.32 1
R1# 2.2.2.32(Connected)
 unresolved(Connected)
 Client list: sharp(fd 50)

-Now a .31/32 comes which will resolve over .32 route, but as per the current logic,
this will trigger the NHT check, in turn making the NHT unresolved.

-With fix, NHT should stay in resolved state as long as the valid static or connected route stays installed

Fix:
====
-While resolving nexthops, walk up the tree from the most-specific match,
walk up the tree without any ZEBRA_NHT_CONNECTED check.

Co-authored-by: Vishal Dhingra <vdhingra@vmware.com>
Co-authored-by: Kantesh Mundaragi <kmundaragi@vmware.com>
Signed-off-by: Iqra Siddiqui <imujeebsiddi@vmware.com>