nexthop: add support for notifiers

This patch adds nexthop add/del notifiers. To be used by
vxlan driver in a later patch. Could possibly be used by
switchdev drivers in the future.

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