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

This reverts commit c9af9fb6.

We need to block and truncate all pages in order to reliably invalidate
them.  Otherwise, we could:

 - have some uptodate pages in the cache
 - queue an invalidate
 - write(2) locks some pages
 - invalidate_work skips them
 - write(2) only overwrites part of the page
 - page now dirty and uptodate
 -> partial leakage of invalidated data

It's not entirely clear why we started skipping locked pages in the first
place.  I just ran this through fsx and didn't see any problems.

Signed-off-by: Sage Weil <sage@newdream.net>