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

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

commit 0260ae0c246e627a06d487d3a52e7fdb5eb23f2c
"net IPv6 : Fix broken IPv6 routing table after loopback down-up"
allocates addrconf router for ipv6 address when lo device up.
but commit 9bd7162b0e4ef2048a6ec36e4849c8560464d822
"ipv6:don't call addrconf_dst_alloc again when enable lo" breaks
this behavior.

Since the addrconf router is moved to the garbage list when
lo device down, we should release this router and rellocate
a new one for ipv6 address when lo device up.

This patch solves bug 67951 on bugzilla
https://bugzilla.kernel.org/show_bug.cgi?id=67951

change from v1:
use ip6_rt_put to repleace ip6_del_rt, thanks Hannes!
change code style, suggested by Sergei.

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>