inet: add a schedule point in inet_twsk_purge()

On a large hash table, we can easily spend seconds to
walk over all entries. Add a cond_resched() to yield
cpu if necessary.
Signed-off-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/inet_timewait_sock.c

@@ -487,6 +487,7 @@ void inet_twsk_purge(struct inet_hashinfo *hashinfo,
 	for (slot = 0; slot <= hashinfo->ehash_mask; slot++) {
 		struct inet_ehash_bucket *head = &hashinfo->ehash[slot];
 restart_rcu:
+		cond_resched();
 		rcu_read_lock();
 restart:
 		sk_nulls_for_each_rcu(sk, node, &head->chain) {