ipstats: Add a group "afstats", subgroup "mpls"

ipstats: Add a group "afstats", subgroup "mpls"

Add a new group, "afstats", for showing counters from the
IFLA_STATS_AF_SPEC nest, and a subgroup "mpls" for the AF_MPLS
specifically.

For example:

 # ip -n ns0-NrdgY9sx stats show dev veth01 group afstats
 3: veth01: group afstats subgroup mpls
     RX: bytes packets errors dropped noroute
             0       0      0       0       0
     TX: bytes packets errors dropped
           108       1      0       0

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