md: get sysfs entry after redundancy attr group create

"sync_completed" and "degraded" belongs to redundancy attr group,
it was not exist yet when md device was created.

Reported-by: kernel test robot <rong.a.chen@intel.com>
Fixes: e1a86dbb ("md: fix deadlock causing by sysfs_notify")
Signed-off-by: Junxiao Bi <junxiao.bi@oracle.com>
Signed-off-by: Song Liu <songliubraving@fb.com>