m68k: Use BUILD_BUG for passing invalid sizes to get_user/put_user

Simplify the handling a bit by using the common helper instead of
referencing undefined symbols.

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/20210916070405.52750-4-hch@lst.de


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