ipv6: don't disable interface if last ipv6 address is removed

The reason behind this change is that as soon as we delete
the last ipv6 address of an interface we also lose the
/proc/sys/net/ipv6/conf/<interface> directory. This seems to be a
usability problem for me.

I don't see any reason why we should shutdown ipv6 on that interface in
such cases.

CRs-fixed: 691450
Change-Id: Ic926a4ee8b54f4687a58e067d11e7dd4ea41f524
Cc: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Git-commit: 876fd05ddbae03166e7037fca957b55bb3be6594
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>