LU-9356 osd-ldiskfs: add blk_plug when do bio

LU-9356 osd-ldiskfs: add blk_plug when do bio

During 16MB bulk RPC I/O evaluation on rhel7, due to kernel
BIO_MAX_PAGES (256) limit, the 16MB IO is divided into 16 1MB
I/O submitting to underly block device one by one. And we found
that the SFA disk driver got lots of 1MB IOs.
To optimize the performance, this patch introduces blk_plug into
osd-ldiskfs when do bio, before submit IOs, it calls blk_start_plug,
after submit all 16MB IOs, calls blk_finish_plug, so that the 16MB
bulk IO will have more change to merge in the block evelvator
scheduler layer.

Signed-off-by: Qian Yingjin <qian@ddn.com>
Change-Id: If26db9f85baf97bc441cc4ad19d5c9f97bd3d7e5
Reviewed-on: https://review.whamcloud.com/26697
Tested-by: Jenkins
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Li Xi <lixi@ddn.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>