sh: Fix up redundant cache flushing for PAGE_SIZE > 4k.

If PAGE_SIZE is presently over 4k we do a lot of extra flushing given
that we purge the cache 4k at a time. Make it explicitly 4k per
iteration, rather than iterating for PAGE_SIZE before looping over again.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>