Ceph » CephFS

Add backtraces to each file object, as described at http://www.spinics.net/lists/ceph-devel/msg11872.html. This ticket includes the MDS asynchronously updating backtraces on creates and renames, as well as the "sentinel object" infrastructure, but not the lookup infrastructure.

For this initial implementation, the MDS should add the backtrace on file create (so we end up with a sentinel for all objects whose pool gets changed).