[BRIDGE]: Reduce frequency of forwarding cleanup timer in bridge.

The bridge cleanup timer is fired 10 times a second for timers that
are at least 15 seconds ahead in time and that are not critical to be
cleaned asap.

This patch calculates the next time to run the timer as the minimum of
all timers or a minimum based on the current state.

Signed-off-by: Baruch Even <baruch@ev-en.org>
Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>