[SCSI] fcoe: Fix module ref count bug by adding NETDEV UNREGISTER handling

Fixes reference counting on fcoe_instance and net_device, and adds
NETDEV_UNREGISTER notifier handling so that you can unload network drivers.
FCoE no longer increments the module use count for the network driver.

On an NETDEV_UNREGISTER event, destroying the FCoE instance is deferred to a
workqueue context to avoid RTNL deadlocks.

Based in part by an earlier patch from John Fastabend

John's patch description:
Currently, the netdev module ref count is not decremented with module_put()
when the module is unloaded while fcoe instances are present. To fix this
removed reference count on netdev module completely and added functionality to
netdev event handling for NETDEV_UNREGISTER events.

This allows fcoe to remove devices cleanly when the netdev module is unloaded
so we no longer need to hold a reference count for the netdev module.
Signed-off-by: Chris Leech <christopher.leech@intel.com>
Signed-off-by: Robert Love <robert.w.love@intel.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>

drivers/scsi/fcoe/fcoe.h

@@ -93,9 +93,11 @@ struct fcoe_interface {
  */
 struct fcoe_port {
 	struct fcoe_interface *fcoe;
+	struct fc_lport *lport;
 	struct sk_buff_head fcoe_pending_queue;
 	u8	fcoe_pending_queue_active;
 	struct timer_list timer;		/* queue timer */
+	struct work_struct destroy_work;	/* to prevent rtnl deadlocks */
 };
 
 #define fcoe_from_ctlr(fip) container_of(fip, struct fcoe_interface, ctlr)