*: modify VRF_CONFIGURED flag only in VRF NB layer

*: modify VRF_CONFIGURED flag only in VRF NB layer

This is to fix the crash reproduced by the following steps:

* ip link add red type vrf table 1

  Creates VRF.

* vtysh -c "conf" -c "vrf red"

  Creates VRF NB node and marks VRF as configured.

* ip route 1.1.1.0/24 2.2.2.2 vrf red
* no ip route 1.1.1.0/24 2.2.2.2 vrf red
  (or similar l3vni set/unset in zebra)

  Marks VRF as NOT configured.

* ip link del red

  VRF is deleted, because it is marked as not configured, but NB node
  stays.

Subsequent attempt to configure something in the VRF leads to a crash
because of the stale pointer in NB layer.

Fixes #8357.

Signed-off-by: Igor Ryzhov <iryzhov@nfware.com>