net: generic fdb support for drivers without ndo_fdb_<op>

If the driver does not support the ndo_op use the generic
handler for it. This should work in the majority of cases.
Eventually the fdb_dflt_add call gets translated into a
__dev_set_rx_mode() call which should handle hardware
support for filtering via the IFF_UNICAST_FLT flag.

Namely IFF_UNICAST_FLT indicates if the hardware can do
unicast address filtering. If no support is available
the device is put into promisc mode.

Signed-off-by: Vlad Yasevich <vyasevic@redhat.com>
Signed-off-by: John Fastabend <john.r.fastabend@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>