LU-4300 ldlm: ELC picks locks in a safer policy

Change the policy of ELC to pick locks that have no dirty pages,
no page in writeback state, and no locked pages.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: Ie73130c4100e3b91c211635f6a72fe5dad994426
Reviewed-on: http://review.whamcloud.com/9175
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Bobi Jam <bobijam@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>