ipv6: Revert 'administrative down' address handling changes.

This reverts the following set of commits:

d1ed113f ("ipv6: remove duplicate neigh_ifdown")
29ba5fed ("ipv6: don't flush routes when setting loopback down")
9d82ca98 ("ipv6: fix missing in6_ifa_put in addrconf")
2de79570 ("ipv6: addrconf: don't remove address state on ifdown if the address is being kept")
8595805a ("IPv6: only notify protocols if address is compeletely gone")
27bdb2ab ("IPv6: keep tentative addresses in hash table")
93fa159a ("IPv6: keep route for tentative address")
8f37ada5 ("IPv6: fix race between cleanup and add/delete address")
84e8b803 ("IPv6: addrconf notify when address is unavailable")
dc2b99f7

 ("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>