sch_cake: fix srchost/dsthost hashing mode

When adding support for using the skb->hash value as the flow hash in CAKE,
I accidentally introduced a logic error that broke the host-only isolation
modes of CAKE (srchost and dsthost keywords). Specifically, the flow_hash
variable should stay initialised to 0 in cake_hash() in pure host-based
hashing mode. Add a check for this before using the skb->hash value as
flow_hash.

Fixes: b0c19ed6 ("sch_cake: Take advantage of skb->hash where appropriate")
Reported-by: NPete Heist <pete@heistp.net>
Tested-by: NPete Heist <pete@heistp.net>
Signed-off-by: NToke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/sched/sch_cake.c

@@ -720,7 +720,7 @@ static u32 cake_hash(struct cake_tin_data *q, const struct sk_buff *skb,
 skip_hash:
 	if (flow_override)
 		flow_hash = flow_override - 1;
-	else if (use_skbhash)
+	else if (use_skbhash && (flow_mode & CAKE_FLOW_FLOWS))
 		flow_hash = skb->hash;
 	if (host_override) {
 		dsthost_hash = host_override - 1;