ipv6: Never schedule DAD timer on dead address

This patch ensures that all places that schedule the DAD timer
look at the address state in a safe manner before scheduling the
timer.  This ensures that we don't end up with pending timers
after deleting an address.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>