spidernet : fix memory leak in spider_net_stop

We forget to call spider_net_free_rx_chain_contents which does the
actual dev_kfree_skb. New skbs are allocated from skbuff_head_cache
on each "ifconfig up" letting the cache grow infinitely.

This patch fixes it.

Signed-off-by: Jens Osterkamp <jens@de.ibm.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>