bgpd: Set NEXT_HOP attribute for EVPN imported routes

bgpd: Set NEXT_HOP attribute for EVPN imported routes

Ensure that when EVPN routes are imported into a VRF as IPv4 routes,
the NEXT_HOP attribute is set. In the absence of this, this attribute
is currently not generated when advertising the route to peers in the
VRF. It is to be noted that the source route (the EVPN route) will only
have the MP_REACH_NLRI attribute that contains the next hop in it.

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>
Reviewed-by:   Mitesh Kanjariya <mitesh@cumulusnetworks.com>
Reviewed-by:   Don Slice <dslice@cumulusnetworks.com>