ipv6: reallocate addrconf router for ipv6 address when lo device up

It fix the bug 67951 on bugzilla
https://bugzilla.kernel.org/show_bug.cgi?id=67951

The patch can't be applied directly, as it' used the function introduced
by "commit 94e187c0" ip6_rt_put(), that patch can't be applied directly
either.

====================

From: Gao feng <gaofeng@cn.fujitsu.com>

commit 33d99113b1102c2d2f8603b9ba72d89d915c13f5 upstream.

This commit don't have a stable tag, but it fix the bug
no reply after loopback down-up.It's very worthy to be
applied to stable 3.4 kernels.

The bug is 67951 on bugzilla
https://bugzilla.kernel.org/show_bug.cgi?id=67951

CC: Sabrina Dubroca <sd@queasysnail.net>
CC: Hannes Frederic Sowa <hannes@stressinduktion.org>
Reported-by: Weilong Chen <chenweilong@huawei.com>
Signed-off-by: Weilong Chen <chenweilong@huawei.com>
Signed-off-by: Gao feng <gaofeng@cn.fujitsu.com>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
[weilong: s/ip6_rt_put/dst_release]
Signed-off-by: Chen Weilong <chenweilong@huawei.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>