nexthop: Add support for resilient nexthop groups

nexthop: Add support for resilient nexthop groups

Add ability to configure resilient nexthop groups and show their current
configuration. Example:

 # ip nexthop add id 10 group 1/2 type resilient buckets 8
 # ip nexthop show id 10
 id 10 group 1/2 type resilient buckets 8 idle_timer 120 unbalanced_timer 0
 # ip -j -p nexthop show id 10
 [ {
         "id": 10,
         "group": [ {
                 "id": 1
             },{
                 "id": 2
             } ],
         "type": "resilient",
         "resilient_args": {
             "buckets": 8,
             "idle_timer": 120,
             "unbalanced_timer": 0
         },
         "flags": [ ]
     } ]

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David Ahern <dsahern@kernel.org>