LU-7990 ofd: add a parameter to adjust preferred brw size

LU-7990 ofd: add a parameter to adjust preferred brw size

Add parameter ofd_device::ofd_brw_size to set the
preferred BRW size of an OST.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Signed-off-by: Gu Zheng <gzheng@ddn.com>
Change-Id: Iab9b26fb1e1c1af8de10d2c96da69151a0a654f5
Reviewed-on: http://review.whamcloud.com/19367
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Reviewed-by: Li Xi <lixi@ddn.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>