LU-8353 mdt: mdt unlink should lock before lookup

LU-8353 mdt: mdt unlink should lock before lookup

MDT unlink should lock parent before lookup child, otherwise the
child may be stale in use later.

Signed-off-by: Lai Siyao <lai.siyao@intel.com>
Change-Id: I70560beba7f689bb183ba83c698babc4ce024ece
Reviewed-on: http://review.whamcloud.com/21088
Tested-by: Jenkins
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>