mds: fix infinite loop of MDCache::populate_mydir().

make MDCache::populate_mydir() only fetch bare-bone stray dirs.
After all stray dirs are populated, call MDCache::scan_stray_dir(),
it fetches incomplete stray dirs.

Fixes: #4405
Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
Merge pull request #702 from ceph/wip-4405

mds: fix infinite loop of MDCache::populate_mydir().

Reviewed-by: Greg Farnum <greg@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>