IPv6: addrconf dad timer unnecessary bh_disable

Timer code runs in bottom half, so there is no need for
using _bh form of locking.  Also check if device is not ready
to avoid race with address that is no longer active.

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