sh: force dcache flush if dcache_dirty bit set.

This too follows the ARM change, given that the issue at hand applies to
all platforms that implement lazy D-cache writeback.

This fixes up the case when a page mapping disappears between the
flush_dcache_page() call (when PG_dcache_dirty is set for the page) and
the update_mmu_cache() call -- such as in the case of swap cache being
freed early. This kills off the mapping test in update_mmu_cache() and
switches to simply testing for PG_dcache_dirty.

Reported-by: Nitin Gupta <ngupta@vflare.org>
Reported-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>