ipv6: always add flag an address that failed DAD with DADFAILED

commit 3d171f3907329d4b1ce31d5ec9c852c5f0269578 upstream.

The userspace needs to know why is the address being removed so that it can
perhaps obtain a new address.

Without the DADFAILED flag it's impossible to distinguish removal of a
temporary and tentative address due to DAD failure from other reasons (device
removed, manual address removal).

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --cc net/ipv6/addrconf.c
Simple merge