mds: properly propagate dirty dirstat to auth inode

Propagate dirty dirstat to freezing auth inode if the inode is
already auth pinned by the Mutation. Otherwiese the dirstat can
be propagated to inode after client changes inode's mtime.

Fixes: #7880
Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
Merge pull request #1548 from ceph/wip-7880

mds: properly propagate dirty dirstat to auth inode

Reviewed-by: Sage Weil <sage@inktank.com>