devlink: Add devlink trap group set and show commands

devlink: Add devlink trap group set and show commands

These commands are similar to the trap set and show commands, but
operate on a trap group and not individual traps. Example:

# devlink trap group set netdevsim/netdevsim10 group l3_drops action trap
# devlink -jps trap group show netdevsim/netdevsim10 group l3_drops
{
    "trap_group": {
        "netdevsim/netdevsim10": [ {
                "name": "l3_drops",
                "generic": true,
                "stats": {
                    "rx": {
                        "bytes": 0,
                        "packets": 0
                    }
                }
            } ]
    }
}

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David Ahern <dsahern@gmail.com>