[IPV6]: order addresses by scope

If IPv6 addresses are ordered by scope, then ipv6_dev_get_saddr() can
break-out of the device addr_list for() loop when the candidate source
address scope is less than the destination address scope.

Signed-off-by: Brian Haley <brian.haley@hp.com>
Acked-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>