staging/lustre/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>
Reviewed-on: http://review.whamcloud.com/9175
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-4300


Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Bobi Jam <bobijam@gmail.com>
Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>