IB: Fix RCU lockdep splats

Commit f2c31e32b3 ("net: fix NULL dereferences in check_peer_redir()")
forgot to take care of infiniband uses of dst neighbours.

Many thanks to Marc Aurele who provided a nice bug report and feedback.

Reported-by: Marc Aurele La France <tsi@ualberta.ca>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Cc: David Miller <davem@davemloft.net>
Cc: <stable@kernel.org>
Signed-off-by: Roland Dreier <roland@purestorage.com>

drivers/infiniband/hw/cxgb4/cm.c

@@ -1594,6 +1594,7 @@ static int pass_accept_req(struct c4iw_dev *dev, struct sk_buff *skb)
 		goto reject;
 	}
 	dst = &rt->dst;
+	rcu_read_lock();
 	neigh = dst_get_neighbour(dst);
 	if (neigh->dev->flags & IFF_LOOPBACK) {
 		pdev = ip_dev_find(&init_net, peer_ip);
@@ -1620,6 +1621,7 @@ static int pass_accept_req(struct c4iw_dev *dev, struct sk_buff *skb)
 		rss_qid = dev->rdev.lldi.rxq_ids[
 			  cxgb4_port_idx(neigh->dev) * step];
 	}
+	rcu_read_unlock();
 	if (!l2t) {
 		printk(KERN_ERR MOD "%s - failed to allocate l2t entry!\n",
 		       __func__);
@@ -1820,6 +1822,7 @@ static int c4iw_reconnect(struct c4iw_ep *ep)
 	}
 	ep->dst = &rt->dst;
 
+	rcu_read_lock();
 	neigh = dst_get_neighbour(ep->dst);
 
 	/* get a l2t entry */
@@ -1856,6 +1859,7 @@ static int c4iw_reconnect(struct c4iw_ep *ep)
 		ep->rss_qid = ep->com.dev->rdev.lldi.rxq_ids[
 			cxgb4_port_idx(neigh->dev) * step];
 	}
+	rcu_read_unlock();
 	if (!ep->l2t) {
 		printk(KERN_ERR MOD "%s - cannot alloc l2e.\n", __func__);
 		err = -ENOMEM;
@@ -2301,6 +2305,7 @@ int c4iw_connect(struct iw_cm_id *cm_id, struct iw_cm_conn_param *conn_param)
 	}
 	ep->dst = &rt->dst;
 
+	rcu_read_lock();
 	neigh = dst_get_neighbour(ep->dst);
 
 	/* get a l2t entry */
@@ -2339,6 +2344,7 @@ int c4iw_connect(struct iw_cm_id *cm_id, struct iw_cm_conn_param *conn_param)
 		ep->retry_with_mpa_v1 = 0;
 		ep->tried_with_mpa_v1 = 0;
 	}
+	rcu_read_unlock();
 	if (!ep->l2t) {
 		printk(KERN_ERR MOD "%s - cannot alloc l2e.\n", __func__);
 		err = -ENOMEM;