LU-7990 clio: revise readahead to support 16MB IO

LU-7990 clio: revise readahead to support 16MB IO

The major work is to adjust read ahead policy to issue
read ahead RPC by underlying RPC size

Test case is backport from Andrew Perepechko's
"LU-7140 llite: too few 4 MiB RPCs from readahead"
http://review.whamcloud.com/16374

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Signed-off-by: Gu Zheng <gzheng@ddn.com>
Change-Id: Ib25ff37712389cca7fcee86996124e7c9dd4ec9a
Reviewed-on: http://review.whamcloud.com/19368
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Li Xi <lixi@ddn.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>