fcoe: convert to kworker

The driver creates its own per-CPU threads which are updated based on
CPU hotplug events. It is also possible to use kworkers and remove some
of the kthread infrastrucure.

The code checked ->thread to decide if there is an active per-CPU
thread. By using the kworker infrastructure this is no longer
possible (or required). The thread pointer is saved in `kthread' instead
of `thread' so anything trying to use thread is caught by the
compiler. Currently only the bnx2fc driver is using struct fcoe_percpu_s
and the kthread member.

After a CPU went offline, we may still enqueue items on the "offline"
CPU. This isn't much of a problem. The work will be done on a random
CPU. The allocated crc_eof_page page won't be cleaned up. It is probably
expected that the CPU comes up at some point so it should not be a
problem. The crc_eof_page memory is released of course once the module
is removed.

This patch was only compile-tested due to -ENODEV.

Cc: Vasu Dev <vasu.dev@intel.com>
Cc: "James E.J. Bottomley" <jejb@linux.vnet.ibm.com>
Cc: "Martin K. Petersen" <martin.petersen@oracle.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: fcoe-devel@open-fcoe.org
Cc: linux-scsi@vger.kernel.org
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Tested-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

include/scsi/libfcoe.h

@@ -319,14 +319,16 @@ struct fcoe_transport {
 
 /**
  * struct fcoe_percpu_s - The context for FCoE receive thread(s)
- * @thread:	    The thread context
+ * @kthread:	    The thread context (used by bnx2fc)
+ * @work:	    The work item (used by fcoe)
  * @fcoe_rx_list:   The queue of pending packets to process
  * @page:	    The memory page for calculating frame trailer CRCs
  * @crc_eof_offset: The offset into the CRC page pointing to available
  *		    memory for a new trailer
  */
 struct fcoe_percpu_s {
-	struct task_struct *thread;
+	struct task_struct *kthread;
+	struct work_struct work;
 	struct sk_buff_head fcoe_rx_list;
 	struct page *crc_eof_page;
 	int crc_eof_offset;