USB: make usb-skeleton honor O_NONBLOCK in write path

usb:usb-skeleton: honor O_NONBLOCK in write path

nonblocking writes are allowed by using down_trylock if necessary
to reserve an URB
Signed-off-by: NOliver Neukum <oliver@neukum.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

drivers/usb/usb-skeleton.c

@@ -399,10 +399,17 @@ static ssize_t skel_write(struct file *file, const char *user_buffer, size_t cou
 		goto exit;
 
 	/* limit the number of URBs in flight to stop a user from using up all RAM */
+	if (!file->f_flags & O_NONBLOCK) {
 		if (down_interruptible(&dev->limit_sem)) {
 			retval = -ERESTARTSYS;
 			goto exit;
 		}
+	} else {
+		if (down_trylock(&dev->limit_sem)) {
+			retval = -EAGAIN;
+			goto exit;
+		}
+	}
 
 	spin_lock_irq(&dev->err_lock);
 	if ((retval = dev->errors) < 0) {