bgpd: Ensure correct checks for EVPN route import

bgpd: Ensure correct checks for EVPN route import

In a situation where a VRF has configured route targets for importing
EVPN routes, this configuration may exist prior to the VRF being
ready to have EVPN routes installed into it - e.g., still missing
the L3VNI configuration or associated interface information. Ensure
that this is taken into account during EVPN route import and unimport.
Without this fix, EVPN routes would end up being prematurely imported
into the VRF routing table and consequently installed as inactive
(because the nexthop information would be incorrect when BGP informs
zebra).

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>