zebra: fix wrong nexthop check for kernel routes

zebra: fix wrong nexthop check for kernel routes

When changing one interface's vrf, the kernel routes are wrongly kept
in old vrf.  Finally, the forwarding table in that old vrf can't forward
traffic correctly for those residual entries.

Follow these steps to make this problem happen:
( Firstly, "x1" interface of default vrf is with address of "6.6.6.6/24". )

```
anlan# ip route add 4.4.4.0/24 via 6.6.6.8 dev x1
anlan# ip link add vrf1 type vrf table 1
anlan# ip link set vrf1 up
anlan# ip link set x1 master vrf1
```

Then check `show ip route`, the route of "4.4.4.0/24" is still selected
in default vrf.

If the interface goes down, the kernel routes will be reevaluated.  Those
kernel routes with active interface of nexthop can be kept no change, it
is a fast path.  Otherwise, it enters into slow path to do careful examination
on this nexthop.

After the interface's vrf had been changed into new vrf, the down message of
this interface came.  It means the interface is not in old vrf although it
still exists during that checking, so the kernel routes should be dropped
after this nexthop matching against a default route in slow path. But, in
current code they are wrongly kept in fast path for not checking vrf.

So, modified the checking active nexthop with vrf comparision for the interface
during reevaluation.

Signed-off-by: anlan_cs <vic.lan@pica8.com>