tcp_metrics: optimize tcp_metrics_flush_all()

This is inspired by several syzbot reports where
tcp_metrics_flush_all() was seen in the traces.

We can avoid acquiring tcp_metrics_lock for empty buckets,
and we should add one cond_resched() to break potential long loops.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/ipv4/tcp_metrics.c

@@ -898,11 +898,13 @@ static void tcp_metrics_flush_all(struct net *net)
 	unsigned int row;
 
 	for (row = 0; row < max_rows; row++, hb++) {
-		struct tcp_metrics_block __rcu **pp;
+		struct tcp_metrics_block __rcu **pp = &hb->chain;
 		bool match;
 
+		if (!rcu_access_pointer(*pp))
+			continue;
+
 		spin_lock_bh(&tcp_metrics_lock);
-		pp = &hb->chain;
 		for (tm = deref_locked(*pp); tm; tm = deref_locked(*pp)) {
 			match = net ? net_eq(tm_net(tm), net) :
 				!refcount_read(&tm_net(tm)->ns.count);
@@ -914,6 +916,7 @@ static void tcp_metrics_flush_all(struct net *net)
 			}
 		}
 		spin_unlock_bh(&tcp_metrics_lock);
+		cond_resched();
 	}
 }