binfmt_flat: clean up create_flat_tables() and stack accesses

In addition to better code clarity, this brings proper usage of
user memory accessors everywhere the stack is touched. This is essential
for making this work on MMU systems.

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>