staging: lustre: hsm: Add support to drop all pages for ll_data_version

This will be used by HSM release to get data version and drop all
caching pages from all clients, before sending IT_RELEASE close
REQ to MDT.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-3647
Reviewed-on: http://review.whamcloud.com/6794


Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Aurelien Degremont <aurelien.degremont@cea.fr>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>