[IPV6]: Call inet6addr_chain notifiers on link down

Currently if the link is brought down via ip link or ifconfig down,
the inet6addr_chain notifiers are not called even though all
the addresses are removed from the interface.  This caused SCTP
to add duplicate addresses to it's list.

Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>