USB: gadgetfs: fix copy_to_user while holding spinlock

The gadgetfs driver as a long-outstanding FIXME, regarding a call of
copy_to_user() made while holding a spinlock.  This patch fixes the
issue by dropping the spinlock and using the dev->udc_usage mechanism
introduced by another recent patch to guard against status changes
while the lock isn't held.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Reported-by: Andrey Konovalov <andreyknvl@google.com>
CC: <stable@vger.kernel.org>
Acked-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/gadget/legacy/inode.c

@@ -983,11 +983,14 @@ ep0_read (struct file *fd, char __user *buf, size_t len, loff_t *ptr)
 				retval = -EIO;
 			else {
 				len = min (len, (size_t)dev->req->actual);
-// FIXME don't call this with the spinlock held ...
+				++dev->udc_usage;
+				spin_unlock_irq(&dev->lock);
 				if (copy_to_user (buf, dev->req->buf, len))
 					retval = -EFAULT;
 				else
 					retval = len;
+				spin_lock_irq(&dev->lock);
+				--dev->udc_usage;
 				clean_req (dev->gadget->ep0, dev->req);
 				/* NOTE userspace can't yet choose to stall */
 			}