bridge/fdb: add flag/indication for FDB entry synced from offload device

bridge/fdb: add flag/indication for FDB entry synced from offload device

Add NTF_EXT_LEARNED flag to neigh flags to indicate FDB entry learned by
device has been learned externally to bridge FDB.  For these entries,
add "external" annotation in bridge fdb show output:

  00:02:00:00:03:00 dev swp2 used 2/2 master br0 external
  00:02:00:00:03:00 dev swp2 self permanent

In the example above, 00:02:00:00:03:00 is shown twice on dev swp2.  The
first entry if from the bridge (master) and is marked as "external" by
the offload device.  The second entry is from the brport offload device (self),
and was learned by the device.

Signed-off-by: Scott Feldman <sfeldma@gmail.com>
Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>