9p: fix file headers

- add missing SPDX-License-Identifier
- remove (sometimes incorrect) file name from file header

Link: http://lkml.kernel.org/r/20211102134608.1588018-2-dominique.martinet@atmark-techno.com
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>

net/9p/protocol.c

@@ -1,7 +1,5 @@
 // SPDX-License-Identifier: GPL-2.0-only
 /*
- * net/9p/protocol.c
- *
  * 9P Protocol Support Code
  *
  *  Copyright (C) 2008 by Eric Van Hensbergen <ericvh@gmail.com>