usb: usbtest: use irqsave() in USB's complete callback

The USB completion callback does not disable interrupts while acquiring
the lock. We want to remove the local_irq_disable() invocation from
__usb_hcd_giveback_urb() and therefore it is required for the callback
handler to disable the interrupts while acquiring the lock.
The callback may be invoked either in IRQ or BH context depending on the
USB host controller.
Use the _irqsave() variant of the locking primitives.

Cc: Greg Kroah-Hartman <[e-mail]>
Acked-by: Alan Stern <[e-mail]>
Signed-off-by: Sebastian Andrzej Siewior <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/usb/misc/usbtest.c

···
 	struct usb_ctrlrequest	*reqp;
 	struct subcase		*subcase;
 	int			status = urb->status;
+	unsigned long		flags;
 
 	reqp = (struct usb_ctrlrequest *)urb->setup_packet;
 	subcase = container_of(reqp, struct subcase, setup);
 
-	spin_lock(&ctx->lock);
+	spin_lock_irqsave(&ctx->lock, flags);
 	ctx->count--;
 	ctx->pending--;
 
···
 	/* signal completion when nothing's queued */
 	if (ctx->pending == 0)
 		complete(&ctx->complete);
-	spin_unlock(&ctx->lock);
+	spin_unlock_irqrestore(&ctx->lock, flags);
 }
 
 static int
···
 static void complicated_callback(struct urb *urb)
 {
 	struct transfer_context	*ctx = urb->context;
+	unsigned long flags;
 
-	spin_lock(&ctx->lock);
+	spin_lock_irqsave(&ctx->lock, flags);
 	ctx->count--;
 
 	ctx->packet_count += urb->number_of_packets;
···
 		complete(&ctx->done);
 	}
 done:
-	spin_unlock(&ctx->lock);
+	spin_unlock_irqrestore(&ctx->lock, flags);
 }
 
 static struct urb *iso_alloc_urb(