net-rps: fixes for rps flow limit

Caught by sparse:
- __rcu: missing annotation to sd->flow_limit
- __user: direct access in cpumask_scnprintf

Also
- add endline character when printing bitmap if room in buffer
- avoid bucket overflow by reducing FLOW_LIMIT_HISTORY

The last item warrants some explanation. The hashtable buckets are
subject to overflow if FLOW_LIMIT_HISTORY is larger than or equal
to bucket size, since all packets may end up in a single bucket. The
current (rather arbitrary) history value of 256 happens to match the
buffer size (u8).

As a result, with a single flow, the first 128 packets are accepted
(correct), the second 128 packets dropped (correct) and then the
history[] array has filled, so that each subsequent new packet
causes an increment in the bucket for new_flow plus a decrement
for old_flow: a steady state.

This is fine if packets are dropped, as the steady state goes away
as soon as a mix of traffic reappears. But, because the 256th packet
overflowed the bucket to 0: no packets are dropped.

Instead of explicitly adding an overflow check, this patch changes
FLOW_LIMIT_HISTORY to never be able to overflow a single bucket.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
(first item)

Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -1840,7 +1840,7 @@ static inline int unregister_gifconf(unsigned int family)
 }
 
 #ifdef CONFIG_NET_FLOW_LIMIT
-#define FLOW_LIMIT_HISTORY	(1 << 8)	/* must be ^2 */
+#define FLOW_LIMIT_HISTORY	(1 << 7)  /* must be ^2 and !overflow buckets */
 struct sd_flow_limit {
 	u64			count;
 	unsigned int		num_buckets;
@@ -1883,7 +1883,7 @@ struct softnet_data {
 	struct napi_struct	backlog;
 
 #ifdef CONFIG_NET_FLOW_LIMIT
-	struct sd_flow_limit	*flow_limit;
+	struct sd_flow_limit __rcu *flow_limit;
 #endif
 };