LU-6473 mdt: enqueue lookup lock on the parent MDT

LU-6473 mdt: enqueue lookup lock on the parent MDT

Because the parent MDT will control the LOOKUP lock,
so Unlink and migration should enqueue the LOOKUP lock
on the parent MDT.

Signed-off-by: wang di <di.wang@intel.com>
Change-Id: I68fa328dc0e5f255b6a5b99a22db90a722c415fa
Reviewed-on: http://review.whamcloud.com/14495
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>