staging/lustre/ldlm: Hold lock when clearing flag

This patch moves lock's skip flag clearing from lru-delete to
lru-add code to prevent clearing lock's flag without resource lock
protection.

Signed-off-by: Li Xi <lixi@ddn.com>
Signed-off-by: Bobi Jam <bobijam.xu@intel.com>
Reviewed-on: http://review.whamcloud.com/8772
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-4269


Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>