mlxsw: Add support for IFLA_OFFLOAD_XSTATS_L3_STATS

Spectrum machines support L3 stats by binding a counter to a RIF, a
hardware object representing a router interface. Recognize the netdevice
notifier events, NETDEV_OFFLOAD_XSTATS_*, to support enablement,
disablement, and reporting back to core.

As a netdevice gains a RIF, if L3 stats are enabled, install the counters,
and ping the core so that a userspace notification can be emitted.

Similarly, as a netdevice loses a RIF, push the as-yet-unreported
statistics to the core, so that they are not lost, and ping the core to
emit userspace notification.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>