binfmt_flat: assorted cleanups

Remove excessive casts, do some code grouping, fix most important
checkpatch.pl complaints, etc.

No functional changes.
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>