From: Hugh Dickins <hugh@veritas.com>

Most architectures (like i386) do nothing in flush_dcache_page, or don't scan
i_mmap in flush_dcache_page, so don't need flush_dcache_mmap_lock to do
anything: define it and flush_dcache_mmap_unlock away.  Noticed arm26, cris,
h8300 still defining flush_page_to_ram: delete it again.