ipv6: don't flush routes when setting loopback down

When loopback device is being brought down, then keep the route table
entries because they are special. The entries in the local table for
linklocal routes and ::1 address should not be purged.

This is a sub optimal solution to the problem and should be replaced
by a better fix in future.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>