devlink: Add devlink trap policer set and show commands

devlink: Add devlink trap policer set and show commands

The trap policer set command allows the user to set the parameters of
the packet trap policer, such as rate and burst size. Example:

# devlink trap policer set netdevsim/netdevsim10 policer 1 rate 1000 burst 32

The trap policer show command allows the user to get the current
parameters of an individual policer or a dump of all policers in case
one is not specified. When '-s' is specified the policer's statistics
are shown. Example:

# devlink -jps trap policer show netdevsim/netdevsim10 policer 1
{
    "trap_policer": {
        "netdevsim/netdevsim10": [ {
                "policer": 1,
                "rate": 1000,
                "burst": 32,
                "stats": {
                    "rx": {
                        "dropped": 53
                    }
                }
            } ]
    }
}

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