bgpd: Do not perform "connected" check for EVPN nexthop

bgpd: Do not perform "connected" check for EVPN nexthop

This changeset follows the PR
https://github.com/FRRouting/frr/pull/5334

Above PR adds nexthop tracking support for EVPN RT-5 nexthops.
This route is marked VALID only if the BGP route has a valid nexthop.

If the EVPN peer is an EBGP pee and "disable_connected_check" flag is not set,
"connected" check is performed for the EVPN nexthop.
But, usually EVPN nexthop is not the BGP peering address, but the VTEP address.
Also, NEXTHOP_UNCHANGED flag is enabled by default for EVPN.
As a result, in a common deployment for EVPN, EVPN nexthop is not connected.

Thus, adding a fix to remove the "connected" check for EVPN nexthops.

Signed-off-by: Ameya Dharkar <adharkar@vmware.com>