MRP-1644 ost: fix ost_brw_write() cleanup path

obd_commitrw must be called in all cases
if obd_preprw() was successfull.

Change-Id: Ie523b0cfea30bc55ecd58db0849b1b005a393fc7
Signed-off-by: Andriy Skulysh <Andriy_Skulysh@xyratex.com>
Reviewed-on: http://morpheus.xyus.xyratex.com:8443/gerrit/1511
Tested-by: Jenkins
Reviewed-by: Andrew Perepechko <Andrew_Perepechko@xyratex.com>
Reviewed-by: Alexey Lyashkov <Alexey_Lyashkov@xyratex.com>
Tested-by: Elena Gryaznova <Elena_Gryaznova@xyratex.com>
Reviewed-by: Vitaly Fertman <Vitaly_Fertman@xyratex.com>