bgpd: Use BGP instance to derive the VRF for route uninstall

bgpd: Use BGP instance to derive the VRF for route uninstall

When uninstalling routes from zebra, ensure that the BGP instance for
which processing is being done is used to derive the VRF. It is incorrect
to derive the VRF from the peer when dealing with scenarios like VRF route
leaking, EVPN symmetric/external routing etc., where the peer which sourced
the route could belong to a different VRF.

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>
Reviewed-by:   Donald Sharp <sharpd@cumulusnetworks.com>
Reviewed-by:   Mitesh Kanjariya <mitesh@cumulusnetworks.com>
Ticket: CM-18413
Reviewed By: CCR-6778
Testing Done: Manual testing of BGP route withdraw/delete, bgp-min