qeth: Fix crash on initial MTU size change

When the initial MTU size is changed prior to any activity on the device
(e.g. by attaching a z/VM vNIC already configured in Linux to a guestLAN),
we call dev_kfree_skb_irq(NULL) which results in a kernel panic.
Adding a proper check for NULL pointers to address this issue.
Signed-off-by: Stefan Raspl <raspl@linux.vnet.ibm.com>
Signed-off-by: Frank Blaschka <blaschka@linux.vnet.ibm.com>
Reviewed-by: Ursula Braun <braunu@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/s390/net/qeth_core_main.c

@@ -1282,8 +1282,10 @@ static void qeth_free_qdio_buffers(struct qeth_card *card)
 
 	qeth_free_cq(card);
 	cancel_delayed_work_sync(&card->buffer_reclaim_work);
-	for (j = 0; j < QDIO_MAX_BUFFERS_PER_Q; ++j)
+	for (j = 0; j < QDIO_MAX_BUFFERS_PER_Q; ++j) {
+		if (card->qdio.in_q->bufs[j].rx_skb)
 			dev_kfree_skb_any(card->qdio.in_q->bufs[j].rx_skb);
+	}
 	kfree(card->qdio.in_q);
 	card->qdio.in_q = NULL;
 	/* inbound buffer pool */