IPoIB: Fix oops if xmit is called when priv->broadcast is NULL

If a port goes down, ipoib_ib_dev_down() is invoked -- which flushes
the mcasts (clearing priv->broadcast) and clearing the path record
cache.  If ipoib_start_xmit() is then invoked (before the broadcast
group is rejoined), a kernel oops results from attempting to access
priv->broadcast, which is still unset.

Returning NULL from path_rec_create() if priv->broadcast is NULL is a
harmless way of bypassing the problem -- the offending packet is
simply discarded "without prejudice."
Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>

drivers/infiniband/ulp/ipoib/ipoib_main.c

@@ -460,6 +460,9 @@ static struct ipoib_path *path_rec_create(struct net_device *dev, void *gid)
 	struct ipoib_dev_priv *priv = netdev_priv(dev);
 	struct ipoib_path *path;
 
+	if (!priv->broadcast)
+		return NULL;
+
 	path = kzalloc(sizeof *path, GFP_ATOMIC);
 	if (!path)
 		return NULL;