m68k: Document that access_ok is broken for !CONFIG_CPU_HAS_ADDRESS_SPACES

Document that access_ok is completely broken for coldfire and friends at
the moment.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Michael Schmitz <schmitzmic@gmail.com>
Tested-by: Michael Schmitz <schmitzmic@gmail.com>
Link: https://lore.kernel.org/r/20210916070405.52750-2-hch@lst.de


Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>