rocker: implement IPv4 fib offloading

The driver implements ndo_switch_fib_ipv4_add/del ops to add/del/mod IPv4
routes to/from switchdev device.  Once a route is added to the device, and the
route's nexthops are resolved to neighbor MAC address, the device will forward
matching pkts rather than the kernel.  This offloads the L3 forwarding path
from the kernel to the device.  Note that control and management planes are
still mananged by Linux; only the data plane is offloaded.  Standard routing
control protocols such as OSPF and BGP run on Linux and manage the kernel's FIB
via standard rtm netlink msgs...nothing changes here.

A new hash table is added to rocker to track neighbors.  The driver listens for
neighbor updates events using netevent notifier NETEVENT_NEIGH_UPDATE.  Any ARP
table updates for ports on this device are recorded in this table.  Routes
installed to the device with nexthops that reference neighbors in this table
are "qualified".  In the case of a route with nexthops not resolved in the
table, the kernel is asked to resolve the nexthop.

The driver uses fib_info->fib_priority for the priority field in rocker's
unicast routing table.

The device can only forward to pkts matching route dst to resolved nexthops.
Currently, the device only supports single-path routes (i.e. routes with one
nexthop).  Equal Cost Multipath (ECMP) route support will be added in followup
patches.

This patch is driver support for unicast IPv4 routing only.  Followup patches
will add driver and infrastructure for IPv6 routing and multicast routing.
Signed-off-by: Scott Feldman <sfeldma@gmail.com>
Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>