net: suppress RCU lockdep false positive in twsk_net()

Calls to twsk_net() are in some cases protected by reference counting
as an alternative to RCU protection.  Cases covered by reference counts
include __inet_twsk_kill(), inet_twsk_free(), inet_twdr_do_twkill_work(),
inet_twdr_twcal_tick(), and tcp_timewait_state_process().  RCU is used
by inet_twsk_purge().  Locking is used by established_get_first()
and established_get_next().  Finally, __inet_twsk_hashdance() is an
initialization case.

It appears to be non-trivial to locate the appropriate locks and
reference counts from within twsk_net(), so used rcu_dereference_raw().

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/inet_timewait_sock.h

@@ -224,7 +224,9 @@ static inline
 struct net *twsk_net(const struct inet_timewait_sock *twsk)
 {
 #ifdef CONFIG_NET_NS
-	return rcu_dereference(twsk->tw_net);
+	return rcu_dereference_raw(twsk->tw_net); /* protected by locking, */
+						  /* reference counting, */
+						  /* initialization, or RCU. */
 #else
 	return &init_net;
 #endif