sh: Ensure all PG_dcache_dirty pages are written back.

With some of the cache rework an address aliasing optimization was added,
but this managed to fail on certain mappings resulting in pages with
PG_dcache_dirty set never writing back their dcache lines. This patch
reverts to the earlier behaviour of simply always writing back when the
dirty bit is set.

Signed-off-by: Markus Pietrek <Markus.Pietrek@emtrion.de>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>