orangefs: handle zero size write in debugfs

If we write zero bytes to this debugfs file, then it will cause an
underflow when we do copy_from_user(buf, ubuf, count - 1).  Debugfs can
normally only be written to by root so the impact of this is low.
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mike Marshall <hubcap@omnibond.com>

fs/orangefs/orangefs-debugfs.c

@@ -440,6 +440,9 @@ static ssize_t orangefs_debug_write(struct file *file,
 		"orangefs_debug_write: %pD\n",
 		file);
 
+	if (count == 0)
+		return 0;
+
 	/*
 	 * Thwart users who try to jamb a ridiculous number
 	 * of bytes into the debug file...