LU-9470 osc: update timestamps on write only

LU-9470 osc: update timestamps on write only

In osc_io_submit(), we should only update timestamps on write.

Test-parameters: testlist=sanityn.sh,sanityn.sh,sanityn.sh,sanityn.sh,sanityn.sh,sanityn.sh
Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Change-Id: I56ed4c499f3b438018ff5b4cd29d7e84f72e04a6
Reviewed-on: https://review.whamcloud.com/27348
Tested-by: Jenkins
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>