LU-4786 osc: to not pick busy pages for ELC

LU-4786 osc: to not pick busy pages for ELC

This is a combination of commit 154fb1f7 from LU-3321 and commit
bfae5a4e from LU-4300. This patch is only required in b2_5 as the
code is already in 2.6.

Use weigh_ast to decide if a lock covers any pages.
In recovery, weigh_ast will be used to decide if a DLM read lock
covers any locked pages, or it will be canceled instead being
recovered.

The problem with the original implementation is that it attached
each osc_page to an osc_lock also changed lock state to add every
pages for readahead.

Bugzilla-bug-Id: b=16774

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: Ie857a4dd4e235f0c3273f53cbfab9592c2793ac9
Reviewed-on: http://review.whamcloud.com/10795
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Bobi Jam <bobijam@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>