binfmt_flat: use clear_user() rather than memset() to clear .bss

This is needed on systems with a MMU.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Reviewed-by: Greg Ungerer <gerg@linux-m68k.org>
Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>