staging/lustre/osc: add weight function for DLM lock

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.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-on: http://review.whamcloud.com/7894
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-3321


Reviewed-by: Bobi Jam <bobijam@gmail.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>