IPoIB: In unicast_arp_send(), only free newly-created paths

If path_rec_start() returns error, call path_free() only if the path
was newly-created.  If we free an existing path whose valid flag was zero,
(but do not detach it from the list) we cause corruption of the
path list (of which it is a member), and get a kernel crash.

The simplest solution is to not free an existing path -- just leave it
in the list as-is (i.e., with its valid flag cleared).

Thanks to Yossi Etigin of Voltaire for identifying the problem flow
which caused the kernel crash.
Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: Moni Shua <monis@voltaire.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>

drivers/infiniband/ulp/ipoib/ipoib_main.c

@@ -660,8 +660,12 @@ static void unicast_arp_send(struct sk_buff *skb, struct net_device *dev,
 
 	path = __path_find(dev, phdr->hwaddr + 4);
 	if (!path || !path->valid) {
-		if (!path)
+		int new_path = 0;
+
+		if (!path) {
 			path = path_rec_create(dev, phdr->hwaddr + 4);
+			new_path = 1;
+		}
 		if (path) {
 			/* put pseudoheader back on for next time */
 			skb_push(skb, sizeof *phdr);
@@ -669,6 +673,7 @@ static void unicast_arp_send(struct sk_buff *skb, struct net_device *dev,
 
 			if (!path->query && path_rec_start(dev, path)) {
 				spin_unlock_irqrestore(&priv->lock, flags);
+				if (new_path)
 					path_free(dev, path);
 				return;
 			} else