spidernet: null out skb pointer after its been used.

Avoid kernel crash in mm/slab.c due to double-free of pointer.

If the ethernet interface is brought down while there is still
RX traffic in flight, the device shutdown routine can end up
trying to double-free an skb, leading to a crash in mm/slab.c
Avoid the double-free by nulling out the skb pointer.
Signed-off-by: Linas Vepstas <linas@austin.ibm.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>

drivers/net/spider_net.c

@@ -1131,6 +1131,7 @@ spider_net_decode_one_descr(struct spider_net_card *card)
 
 	/* Ok, we've got a packet in descr */
 	spider_net_pass_skb_up(descr, card);
+	descr->skb = NULL;
 	hwdescr->dmac_cmd_status = SPIDER_NET_DESCR_NOT_IN_USE;
 	return 1;