mds: avoid leaking objects when purging file.

The filer implementation does not delete stripe objects that are truncated
to zero. When purging a deleted file, we need to purge stripe objects up to
the max size the file has ever been.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
Merge pull request #708 from ceph/wip-truncate2

Reviewed-by: Greg Farnum <greg@inktank.com>