net: ipv6: Always leave anycast and multicast groups on link down

Default kernel behavior is to delete IPv6 addresses on link
down, which entails deletion of the multicast and the
subnet-router anycast addresses. These deletions do not
happen with sysctl setting to keep global IPv6 addresses on
link down, so every link down/up causes an increment of the
anycast and multicast refcounts. These bogus refcounts may
stop these addrs from being removed on subsequent calls to
delete them. The solution is to leave the groups for the
multicast and subnet anycast on link down for the callflow
when global IPv6 addresses are kept.

Fixes: f1705ec1

 ("net: ipv6: Make address flushing on ifdown optional")
Signed-off-by: Mike Manning <mmanning@brocade.com>
Acked-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>