tcp/dccp: bypass empty buckets in inet_twsk_purge()

TCP ehash table is often sparsely populated.

inet_twsk_purge() spends too much time calling cond_resched().

This patch can reduce time spent in inet_twsk_purge() by 20x.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://lore.kernel.org/r/20240327191206.508114-1-edumazet@google.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv4/inet_timewait_sock.c

@@ -266,12 +266,17 @@ EXPORT_SYMBOL_GPL(__inet_twsk_schedule);
 /* Remove all non full sockets (TIME_WAIT and NEW_SYN_RECV) for dead netns */
 void inet_twsk_purge(struct inet_hashinfo *hashinfo, int family)
 {
+	struct inet_ehash_bucket *head = &hashinfo->ehash[0];
+	unsigned int ehash_mask = hashinfo->ehash_mask;
 	struct hlist_nulls_node *node;
 	unsigned int slot;
 	struct sock *sk;
 
-	for (slot = 0; slot <= hashinfo->ehash_mask; slot++) {
-		struct inet_ehash_bucket *head = &hashinfo->ehash[slot];
+	for (slot = 0; slot <= ehash_mask; slot++, head++) {
+
+		if (hlist_nulls_empty(&head->chain))
+			continue;
+
 restart_rcu:
 		cond_resched();
 		rcu_read_lock();