LU-4931 ofd: use thread buffer for ladvise

LU-4931 ofd: use thread buffer for ladvise

A buffer is allocated everytime when handling willread advice.
This is not efficient, so replace it by using thread buffer
instead.

Signed-off-by: Li Xi <lixi@ddn.com>
Change-Id: I34015ea0658cb116b3348f8cab67128645718cac
Reviewed-on: http://review.whamcloud.com/22489
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>