netfilter: flowtable: fix excessive hw offload attempts after failure

If a flow cannot be offloaded, the code currently repeatedly tries again as
quickly as possible, which can significantly increase system load.
Fix this by limiting flow timeout update and hardware offload retry to once
per second.

Fixes: c07531c ("netfilter: flowtable: Remove redundant hw refresh bit")
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_flow_table_core.c

@@ -335,8 +335,10 @@ void flow_offload_refresh(struct nf_flowtable *flow_table,
 	u32 timeout;
 
 	timeout = nf_flowtable_time_stamp + flow_offload_get_timeout(flow);
-	if (READ_ONCE(flow->timeout) != timeout)
+	if (timeout - READ_ONCE(flow->timeout) > HZ)
 		WRITE_ONCE(flow->timeout, timeout);
+	else
+		return;
 
 	if (likely(!nf_flowtable_hw_offload(flow_table)))
 		return;