LU-4931 ladvise: Add willread advice support for ladvise

This patch adds WILLREAD advice to ladvise framework. OSS will
prefetch data into memory when this hint is provided. It is not
garanteed how long the cached pages will be kept in memory.

Signed-off-by: Li Xi <lixi@ddn.com>
Change-Id: I21394b88a22a8c46ceae7151402341364860ee88
Reviewed-on: http://review.whamcloud.com/12458
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Gu Zheng <gzheng@ddn.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>