[IPV6]: inet6_dev on loopback should be kept until namespace stop.

In the other case it will be destroyed when last address will be removed
from lo inside a namespace. This will break IPv6 in several places. The
most obvious one is ip6_dst_ifdown.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>