net: rtnetlink: Add RTM_SETSTATS

The offloaded HW stats are designed to allow per-netdevice enablement and
disablement. These stats are only accessible through RTM_GETSTATS, and
therefore should be toggled by a RTM_SETSTATS message. Add it, and the
necessary skeleton handler.

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>