ipv6: Revert 'administrative down' address handling changes.

ipv6: Revert 'administrative down' address handling changes.

This reverts the following set of commits:

8f68a1cd34c7f8a8f617c92288413fd4fbe59023 ("ipv6: remove duplicate neigh_ifdown")
a08dcf6f39f95915725ed969f87a8f4a62e4d64e ("ipv6: don't flush routes when setting loopback down")
5d12e4c18df961221ca7d5a5cf5c204af2aaa8cc ("ipv6: fix missing in6_ifa_put in addrconf")
f825dc4edc7a4b9eadf76854d6615be943dae40e ("ipv6: addrconf: don't remove address state on ifdown if the address is being kept")
c48b2a0349314d04fb2b0e07dad7bbabcab1ab53 ("IPv6: only notify protocols if address is compeletely gone")
1e40b890b0778a2c499f3e6a142f86f1fe217f8c ("IPv6: keep tentative addresses in hash table")
d58b1ebb38b49986d59af395d0e3f88408344c73 ("IPv6: keep route for tentative address")
3cf4172bd770c58301159034ff381e4593b08a55 ("IPv6: fix race between cleanup and add/delete address")
0f7d011fd8cf4e5fb8128de149a6065bda856c3a ("IPv6: addrconf notify when address is unavailable")
305b25837607b5bbd77490fdbedef4e235b3dc62 ("IPv6: keep permanent addresses on admin down")

because the core semantic change to ipv6 address handling on ifdown
has broken some things, in particular "disable_ipv6" sysctl handling.

Stephen has made several attempts to get things back in working order,
but nothing has restored disable_ipv6 fully yet.

Reported-by: Eric W. Biederman <ebiederm@xmission.com>
Tested-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>