LU-5727 ldlm: revert the changes for lock canceling policy

LU-5727 ldlm: revert the changes for lock canceling policy

The changes for LRU lock policy was introduced by commit 0a6c6fcd4,
where I was trying to revise the policy to pick locks for canceling.

However, this caused two problems as mentioned in LU-5727. The first
problem is that the lock can only be picked for canceling only if
the number of LRU locks is over preset LRU number AND it's aged; the
second problem is that mdc_cancel_weight() tends to not cancel OPEN
locks, therefore open locks can be kept forever and finally exhausts
memory on the MDT side.

This patch will revert the changes related to LRU policy revision.

This is backported from master:
Lustre-change: http://review.whamcloud.com/12733

LU-5727 ldlm: revert changes to ldlm_cancel_aged_policy()

The changes to ldlm_cancel_aged_policy() intrdouced from LU-4300
(bfae5a) was incorrect. This patch revert this part of changes.

This is backported from master:
Lustre-change: http://review.whamcloud.com/12448
Lustre-commit: e8812867e8c6c9eb62174b370cb818985e3d2180

Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: Ic04f894c8bf54dbbdfb0e3bada5f6e4a367711a5
Reviewed-on: http://review.whamcloud.com/12565
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>