zebra: Fix wrong vrf change procedure

zebra: Fix wrong vrf change procedure

Currently the vrf change procedure for the deleted interface is after
its deletion, it causes problem for upper daemons.

Here is the problem of `bgp`:

After deletion of one **irrelevant** interface in the same vrf, its
`ifindex` is set to 0. And then, the vrf change procedure will send
"ZEBRA_INTERFACE_DOWN" to `bgpd`.

Normally, `bgp_nht_ifp_table_handle()` should igore this message for
no correlation. However, it wrongly matched `ifindex` of 0, and removed
the related routes for the down `bnc`.

Adjust the location of the vrf change procedure to fix this issue.

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