ipnexthop: support for fdb nexthops

ipnexthop: support for fdb nexthops

This patch adds support to add and delete
ecmp nexthops of type fdb. Such nexthops can
be linked to vxlan fdb entries.

$ip nexthop add id 12 via 172.16.1.2 fdb
$ip nexthop add id 13 via 172.16.1.3 fdb
$ip nexthop add id 102 group 12/13 fdb

$bridge fdb add 02:02:00:00:00:13 dev vx10 nhid 102 self

Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David Ahern <dsahern@gmail.com>