rocker: add FDB cleanup timer

Add a timer to each rocker switch to do FDB entry cleanup by ageing out
expired entries.  The timer scheduling algo is copied from the bridge
driver, for the most part, to keep the firing of the timer to a minimum.

Signed-off-by: Scott Feldman <sfeldma@gmail.com>
Acked-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>