neighbour: allow admin to set NTF_ROUTER

This patch allows admin setting of NTF_ROUTER flag
on a neighbour entry. This enables external control
plane (like bgp evpn) to manage neigh entries with
NTF_ROUTER flag.

Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>