bridge: fdb cleanup runs too often

It is common in end-node, non STP bridges to set forwarding
delay to zero; which causes the forwarding database cleanup
to run every clock tick. Change to run only as soon as needed
or at next ageing timer interval which ever is sooner.

Use round_jiffies_up macro rather than attempting round up
by changing value.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>