LU-7655 tests: ost fake write for performance testing

LU-7655 tests: ost fake write for performance testing

Just drop the pages in ofd_commitrw_write(), but we need to maintain
correct file size and always create a transaction so client can pin
those pages in memory until transaction commits.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: Ia9a2af0a159c8969479656d3a7016db3cda71a91
Reviewed-on: http://review.whamcloud.com/5164
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>