dccp: avoid deadlock in dccp_v4_ctl_send_reset

In the prep work I did before enabling BH while handling socket backlog,
I missed two points in DCCP :

1) dccp_v4_ctl_send_reset() uses bh_lock_sock(), assuming BH were
blocked. It is not anymore always true.

2) dccp_v4_route_skb() was using __IP_INC_STATS() instead of
  IP_INC_STATS()

A similar fix was done for TCP, in commit 47dcc20a
("ipv4: tcp: ip_send_unicast_reply() is not BH safe")

Fixes: 7309f882 ("dccp: do not assume DCCP code is non preemptible")
Fixes: 5413d1ba ("net: do not block BH while processing socket backlog")
Signed-off-by: NEric Dumazet <edumazet@google.com>
Reported-by: NDmitry Vyukov <dvyukov@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/dccp/ipv4.c

@@ -462,7 +462,7 @@ static struct dst_entry* dccp_v4_route_skb(struct net *net, struct sock *sk,
 	security_skb_classify_flow(skb, flowi4_to_flowi(&fl4));
 	rt = ip_route_output_flow(net, &fl4, sk);
 	if (IS_ERR(rt)) {
-		__IP_INC_STATS(net, IPSTATS_MIB_OUTNOROUTES);
+		IP_INC_STATS(net, IPSTATS_MIB_OUTNOROUTES);
 		return NULL;
 	}
 
@@ -527,17 +527,19 @@ static void dccp_v4_ctl_send_reset(const struct sock *sk, struct sk_buff *rxskb)
 								 rxiph->daddr);
 	skb_dst_set(skb, dst_clone(dst));
 
+	local_bh_disable();
 	bh_lock_sock(ctl_sk);
 	err = ip_build_and_send_pkt(skb, ctl_sk,
 				    rxiph->daddr, rxiph->saddr, NULL);
 	bh_unlock_sock(ctl_sk);
 
 	if (net_xmit_eval(err) == 0) {
-		DCCP_INC_STATS(DCCP_MIB_OUTSEGS);
-		DCCP_INC_STATS(DCCP_MIB_OUTRSTS);
+		__DCCP_INC_STATS(DCCP_MIB_OUTSEGS);
+		__DCCP_INC_STATS(DCCP_MIB_OUTRSTS);
 	}
+	local_bh_enable();
 out:
-	 dst_release(dst);
+	dst_release(dst);
 }
 
 static void dccp_v4_reqsk_destructor(struct request_sock *req)