mds: properly replay dentry trace for open truncate.

Open request created by ceph_atomic_open() wants dentry trace. But
the corresponding code is missing in Server::do_open_truncate().

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
Merge pull request #1230 from ceph/wip-open-truncate

mds: properly replay dentry trace for open truncate.

Reviewed-by: Sage Weil <sage@inktank.com>