bridge: use time_before() in br_fdb_cleanup()

In br_fdb_cleanup() next_timer and this_timer are in jiffies, so they
should be compared using the time_after() macro.

Signed-off-by: Fabio Checconi <fabio@gandalf.sssup.it>
Signed-off-by: Stephen Hemminger <stephen.hemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>