[patch 1/1][NETNS][IPV6] protect addrconf from loopback registration

[David Miller]

From: "Denis V. Lunev" <den at sw.ru>
Date: Mon, 12 Nov 2007 19:49:03 +0300

> Unregister for a loopback in !init_net is a _valid_ operation and should
> be clean, i.e. without kludges in the path. This is the only way to
> check the ref-counting.

For ipv6 the stack really wants to pin down the loopback
device because we need a valid inet6_dev object to reference
at all times in order to simplify the per-device SNMP
statistic bumping.

When a non-loopback device goes down, we point any existing
references to that device's idev to the loopback one instead.

I really consider taking down the loopback device to be
an invalid operation at least how things are implemented
currently.

There was a suggestion to have a "dummy" device that takes the
place of "point dangling idev refs to loopback's one".  But
some people get upset when statistical events get lost, and
rightly so.  Such a dummy device would either need to be
invisible to the user, or show up and be utterly confusing.