mds: fix stray creation/removal notification

mds: fix stray creation/removal notification

In MDCache::scan_stray_dir(), notify_stray_created() is called for
both null and non-null dentries. This is wrong, it should be called
only for non-null dentries.

Calling notify_stray_created() in MDCache::get_or_create_stray_dentry()
is racy too. MDS can create wrong stray dentry because target dentry
linkage can change before rmdir/rename lock the dentry lock.

Fixes: http://tracker.ceph.com/issues/19630
Signed-off-by: "Yan, Zheng" <zyan@redhat.com>