ipv6: avoid unregistering inet6_dev for loopback

The per netns loopback_dev->ip6_ptr is unregistered and set to
NULL when its mtu is set to smaller than IPV6_MIN_MTU, this
leads to that we could set rt->rt6i_idev NULL after a
rt6_uncached_list_flush_dev() and then crash after another
call.

In this case we should just bring its inet6_dev down, rather
than unregistering it, at least prior to commit 176c39af29bc
("netns: fix addrconf_ifdown kernel panic") we always
override the case for loopback.

Thanks a lot to Andrey for finding a reliable reproducer.

Fixes: 176c39af29bc ("netns: fix addrconf_ifdown kernel panic")
Reported-by: Andrey Konovalov <[email protected]>
Cc: Andrey Konovalov <[email protected]>
Cc: Daniel Lezcano <[email protected]>
Cc: David Ahern <[email protected]>
Signed-off-by: Cong Wang <[email protected]>
Acked-by: David Ahern <[email protected]>
Tested-by: Andrey Konovalov <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

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