mds: use client-provided time stamp for user-visible file metadata

Fixes: #8745
Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
Merge pull request #2075 from ceph/wip-8745

mds: use client-provided time stamp for user-visible file metadata

Reviewed-by: Sage Weil <sage@redhat.com>