LU-3160 clio: don't ignore layout on writeback

LU-3160 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.

Test-Parameters: envdefinitions=DURATION=7200  clientdistro=el6 serverdistro=el6 clientcount=4  osscount=2 mdscount=2 austeroptions=-R failover=true  useiscsi=true testlist=recovery-random-scale
Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: Ib9d0aa581de90711c92db4c631c52f1950ad5b67
Reviewed-on: http://review.whamcloud.com/6154
Tested-by: Hudson
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>