ipstats: Add offload subgroup "l3_stats"

ipstats: Add offload subgroup "l3_stats"

Add into the group "offload" a subgroup "l3_stats" for showing
L3 statistics.

For example:

 # ip stats show dev swp2.200 group offload subgroup l3_stats
 4212: swp2.200: group offload subgroup l3_stats on used on
     RX: bytes packets errors dropped   mcast
          1920      21      1       0       0
     TX: bytes packets errors dropped
           756       9      0       0

 # ip -j stats show dev swp2.200 group offload subgroup l3_stats | jq
 [
   {
     "ifindex": 4212,
     "ifname": "swp2.200",
     "group": "offload",
     "subgroup": "l3_stats",
     "info": {
       "request": true,
       "used": true
     },
     "stats64": {
       "rx": {
         "bytes": 1920,
         "packets": 21,
         "errors": 1,
         "dropped": 0,
         "multicast": 0
       },
       "tx": {
         "bytes": 756,
         "packets": 9,
         "errors": 0,
         "dropped": 0
       }
     }
   }
 ]

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