ipv6: Replace inet6_ifaddr->dead with state

This patch replaces the boolean dead flag on inet6_ifaddr with
a state enum.  This allows us to roll back changes when deleting
an address according to whether DAD has completed or not.

This patch only adds the state field and does not change the logic.

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