LU-8726 osd-ldiskfs: bypass read for benchmarking

LU-8726 osd-ldiskfs: bypass read for benchmarking

The read operation is bypassed, so that Lustre client could get more
than 3GB/s on a single machine. This could be used when benchmarking
client site performance, such as readahead.

Signed-off-by: Li Xi <lixi@ddn.com>
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Change-Id: I518b287bf3c9b26347b43a4b82ef3114f966eb87
Reviewed-on: https://review.whamcloud.com/21648
Reviewed-by: Gu Zheng <gzheng@ddn.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>