ipv6: Add IFA_F_DADFAILED flag

Add IFA_F_DADFAILED flag to denote an IPv6 address that has
failed Duplicate Address Detection, that way tools like
/sbin/ip can be more informative.

3: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qlen 1000
    inet6 2001:db8::1/64 scope global tentative dadfailed
       valid_lft forever preferred_lft forever

Signed-off-by: Brian Haley <brian.haley@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>