ipv6: Do not iterate over all interfaces when finding source address on specific interface.

If outgoing interface is specified and the candidate address is
restricted to the outgoing interface, it is enough to iterate
over that given interface only.

Change-Id: Iec17f955f3a5a0930dc11a8b8916d720d2c5f494
Signed-off-by: YOSHIFUJI Hideaki <hideaki.yoshifuji@miraclelinux.com>
Acked-by: Erik Kline <ek@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>