ipv6: use timer pending

This fixes the bridge reference count problem and cleanups ipv6 FIB
timer management.  Don't use expires field, because it is not a proper
way to test, instead use timer_pending().

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