USB: usb-storage: use kthread_stop() for the control thread

This patch (as923) makes usb-storage's control thread use
kthread_should_stop()/kthread_stop().  The scanning thread can't be
similarly converted until the core kthread implementation allows
threads to call do_exit().

The advantage of this change is that we can now be certain the control
thread has terminated before storage_disconnect() returns.  This will
simplify the locking requirements when autosuspend support is added.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Matthew Dharm <mdharm-usb@one-eyed-alien.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/storage/usb.h

@@ -144,6 +144,7 @@ struct us_data {
 	unsigned char		*sensebuf;	 /* sense data buffer	 */
 	dma_addr_t		cr_dma;		 /* buffer DMA addresses */
 	dma_addr_t		iobuf_dma;
+	struct task_struct	*ctl_thread;	 /* the control thread   */
 
 	/* mutual exclusion and synchronization structures */
 	struct semaphore	sema;		 /* to sleep thread on	    */