For the sake of power saver lovers, use a deferrable timer to fire
rt_check_expire()

As some big routers cache equilibrium depends on garbage collection
done in time, we take into account elapsed time between two
rt_check_expire() invocations to adjust the amount of slots we have to
check.

Based on an initial idea and patch from Tero Kristo
Signed-off-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NTero Kristo <tero.kristo@nokia.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>