m68k: Factor the 8-byte lowlevel {get,put}_user code into helpers

Add new helpers for doing the grunt work of the 8-byte {get,put}_user
routines to allow for better reuse.

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/20210916070405.52750-5-hch@lst.de


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