LU-4931 ladvise: Add dontneed advice support for ladvise

LU-4931 ladvise: Add dontneed advice support for ladvise

This patch addds DONTNEED advice to ladvise framework. OSS will
cleanup the page cache of the file when this hint is provided.

Signed-off-by: Li Xi <lixi@ddn.com>
Signed-off-by: Gu Zheng <gzheng@ddn.com>
Change-Id: If5cf7f3193924ca7cccb96d8d841c0d889469393
Reviewed-on: http://review.whamcloud.com/20203
Reviewed-by: Wang Shilong <wshilong@ddn.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>