m68k: switch to kernel_clone()

The old _do_fork() helper is removed in favor of the new kernel_clone() helper.
The latter adheres to naming conventions for kernel internal syscall helpers.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Kars de Jong <jongk@linux-m68k.org>
Cc: linux-m68k@lists.linux-m68k.org
Link: https://lore.kernel.org/r/20200819104655.436656-5-christian.brauner@ubuntu.com