ipv6: give an IPv6 dev to blackhole_netdev

net-next inclusion
from net-next-v5.17-rc5
commit e5f80fcf
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I4VZN0?from=project-issue

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git/commit/?id=e5f80fcf869a18cc750d6b350bbfac82df292e0b



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IPv6 addrconf notifiers wants the loopback device to
be the last device being dismantled at netns deletion.

This caused many limitations and work arounds.

Back in linux-5.3, Mahesh added a per host blackhole_netdev
that can be used whenever we need to make sure objects no longer
refer to a disappearing device.

If we attach to blackhole_netdev an ip6_ptr (allocate an idev),
then we can use this special device (which is never freed)
in place of the loopback_dev (which can be freed).

This will permit improvements in netdev_run_todo() and other parts
of the stack where had steps to make sure loopback_dev was
the last device to disappear.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Mahesh Bandewar <maheshb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Conflicts:
	net/ipv6/addrconf.c
Signed-off-by: Ziyang Xuan <william.xuanziyang@huawei.com>
Reviewed-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>