inet: make sure to grab rcu_read_lock before using ireq->ireq_opt

Timer handlers do not imply rcu_read_lock(), so my recent fix
triggered a LOCKDEP warning when SYNACK is retransmit.

Lets add rcu_read_lock()/rcu_read_unlock() pairs around ireq->ireq_opt
usages instead of guessing what is done by callers, since it is
not worth the pain.

Get rid of ireq_opt_deref() helper since it hides the logic
without real benefit, since it is now a standard rcu_dereference().

Fixes: 1ad98e9d1b ("tcp/dccp: fix lockdep issue when SYN is backlogged")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dccp/ipv4.c

@@ -493,9 +493,11 @@ static int dccp_v4_send_response(const struct sock *sk, struct request_sock *req
 
 		dh->dccph_checksum = dccp_v4_csum_finish(skb, ireq->ir_loc_addr,
 							      ireq->ir_rmt_addr);
+		rcu_read_lock();
 		err = ip_build_and_send_pkt(skb, sk, ireq->ir_loc_addr,
 					    ireq->ir_rmt_addr,
-					    ireq_opt_deref(ireq));
+					    rcu_dereference(ireq->ireq_opt));
+		rcu_read_unlock();
 		err = net_xmit_eval(err);
 	}