switchdev: implement IPv4 fib ndo wrappers

Flesh out ndo wrappers to call into device driver.  To call into device driver,
the wrapper must interate over route's nexthops to ensure all nexthop devs
belong to the same switch device.  Currently, there is no support for route's
nexthops spanning offloaded and non-offloaded devices, or spanning ports of
multiple offload devices.

Since switch device ports may be stacked under virtual interfaces (bonds and/or
bridges), and the route's nexthop may be on the virtual interface, the wrapper
will traverse the nexthop dev down to the base dev.  It's the base dev that's
passed to the switchdev driver's ndo ops.

Signed-off-by: Scott Feldman <sfeldma@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>