[IPV6]: RFC3484 compliant source address selection

Choose more appropriate source address; e.g.
 - outgoing interface
 - non-deprecated
 - scope
 - matching label

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>