staging/lustre/clio: don't ignore layout on writeback

In some cases such as kernel writeback, we shouldn't ignore the
layout, otherwise, it could race with layout change undergoing.

Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-3160
Lustre-change: http://review.whamcloud.com/6154


Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Peng Tao <tao.peng@emc.com>
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>