RDMA/cxgb4: Fix crash when peer address is 0.0.0.0

When using rping -c -a 0.0.0.0 with iw_cxgb4, the system crashes when
rdma_connect() is called.  ip_dev_find() will return NULL, but pdev is
accessed anyway.

Checking that pdev is NULL and returning -ENODEV prevents the system
from crashing.
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@linux.vnet.ibm.com>
Acked-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>

drivers/infiniband/hw/cxgb4/cm.c

@@ -1593,6 +1593,10 @@ static int import_ep(struct c4iw_ep *ep, __be32 peer_ip, struct dst_entry *dst,
 		struct net_device *pdev;
 
 		pdev = ip_dev_find(&init_net, peer_ip);
+		if (!pdev) {
+			err = -ENODEV;
+			goto out;
+		}
 		ep->l2t = cxgb4_l2t_get(cdev->rdev.lldi.l2t,
 					n, pdev, 0);
 		if (!ep->l2t)