ip6_tunnel: collect_md xmit: Use ip_tunnel_key's provided src address

When using an ip6tnl device in collect_md mode, the xmit methods ignore
the ipv6.src field present in skb_tunnel_info's key, both for route
calculation purposes (flowi6 construction) and for assigning the
packet's final ipv6h->saddr.

This makes it impossible specifying a desired ipv6 local address in the
encapsulating header (for example, when using tc action tunnel_key).

This is also not aligned with behavior of ipip (ipv4) in collect_md
mode, where the key->u.ipv4.src gets used.

Fix, by assigning fl6.saddr with given key->u.ipv6.src.
In case ipv6.src is not specified, ip6_tnl_xmit uses existing saddr
selection code.

Fixes: 8d79266b ("ip6_tunnel: add collect_md mode to IPv6 tunnels")
Signed-off-by: Shmulik Ladkani <shmulik.ladkani@gmail.com>
Reviewed-by: Eyal Birger <eyal.birger@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_tunnel.c

@@ -1113,7 +1113,7 @@ int ip6_tnl_xmit(struct sk_buff *skb, struct net_device *dev, __u8 dsfield,
 			dst = NULL;
 			goto tx_err_link_failure;
 		}
-		if (t->parms.collect_md &&
+		if (t->parms.collect_md && ipv6_addr_any(&fl6->saddr) &&
 		    ipv6_dev_get_saddr(net, ip6_dst_idev(dst)->dev,
 				       &fl6->daddr, 0, &fl6->saddr))
 			goto tx_err_link_failure;
@@ -1255,6 +1255,7 @@ ip4ip6_tnl_xmit(struct sk_buff *skb, struct net_device *dev)
 		key = &tun_info->key;
 		memset(&fl6, 0, sizeof(fl6));
 		fl6.flowi6_proto = IPPROTO_IPIP;
+		fl6.saddr = key->u.ipv6.src;
 		fl6.daddr = key->u.ipv6.dst;
 		fl6.flowlabel = key->label;
 		dsfield =  key->tos;
@@ -1326,6 +1327,7 @@ ip6ip6_tnl_xmit(struct sk_buff *skb, struct net_device *dev)
 		key = &tun_info->key;
 		memset(&fl6, 0, sizeof(fl6));
 		fl6.flowi6_proto = IPPROTO_IPV6;
+		fl6.saddr = key->u.ipv6.src;
 		fl6.daddr = key->u.ipv6.dst;
 		fl6.flowlabel = key->label;
 		dsfield = key->tos;