iomap: fix Invalid License ID

Detected by:

  $ ./scripts/spdxcheck.py
  fs/iomap/Makefile: 1:27 Invalid License ID: GPL-2.0-or-newer

Fixes: 1c230208

 ("iomap: start moving code to fs/iomap/")
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>