bgpd: Fix IP-VRF ext-comm check for MACIP routes

bgpd: Fix IP-VRF ext-comm check for MACIP routes

When validating a MACIP route's IP address,
bgp_evpn_route_add_l3_ecomm_ok checked for a non link-local v6 address
without checking if the v6 address existed.  Since empty v6 addresses
do not fall into the fe80 range, MAC-Only routes were always passing
this check and getting IP-VRF extended communities (RMAC/IP-VRF RT)
added to the routes.
This commit requires ipaddr_v6 to exist AND fall outside of the
link-local range for IP-VRF ext-comms to be added.

Signed-off-by: Trey Aspelund <taspelund@nvidia.com>