ceph: don't truncate dirty pages in invalidate work thread

Instead of truncating the whole range of pages, we skip those
pages that are dirty or in the middle of writeback. Those pages
will be cleared later when the writeback completes.

Signed-off-by: Yehuda Sadeh <yehuda@hq.newdream.net>
Signed-off-by: Sage Weil <sage@newdream.net>