ipv6:fix the outgoing interface selection order in udpv6_sendmsg()

1.When no interface is specified in an IPV6_PKTINFO ancillary data
item, the interface specified in an IPV6_PKTINFO sticky optionis is used.

RFC3542:
6.7. Summary of Outgoing Interface Selection

This document and [RFC-3493] specify various methods that affect the selection of the packet's outgoing interface. This subsection summarizes the ordering among those in order to ensure deterministic behavior.

For a given outgoing packet on a given socket, the outgoing interface is determined in the following order:

1. if an interface is specified in an IPV6_PKTINFO ancillary data item, the interface is used.

2. otherwise, if an interface is specified in an IPV6_PKTINFO sticky option, the interface is used.

---
 net/ipv6/udp.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)

index a6aecf7..e553ab7 100644
@@ -733,6 +733,9 @@ do_udp_sendmsg:
 	}
 
 	if (!fl.oif)
+		fl.oif = np->sticky_pktinfo.ipi6_ifindex;
+
+	if (!fl.oif)
 		fl.oif = sk->sk_bound_dev_if;
 
 	if (msg->msg_controllen) {

1.5.3.8