pbrd: nht only handle if updates if IPV*_IFINDEX nh

pbrd: nht only handle if updates if IPV*_IFINDEX nh

Only handle an interface update in the nexthop tracking code
if the nexthop in question was set with an interface to point
out of. If the nexthop is GW only, the interface update could
be unrelated but have overlapping address space. Let that be
handled elsewhere.

Ex)

```
5.5.5.0/30 dev dummyDoof proto kernel scope link src 5.5.5.1
5.5.5.0/24 dev goofDummy proto kernel scope link src 5.5.5.1
[root@alfred frr-2]# ip ro show table 10000
default via 5.5.5.2 dev dummyDoof proto pbr metric 20
[root@alfred frr-2]# ip link set goofDummy down
[root@alfred frr-2]# ip ro show table 10000
[root@alfred frr-2]# ip link set goofDummy up
[root@alfred frr-2]# ip ro show table 10000

```

Signed-off-by: Stephen Worley <sworley@nvidia.com>