LU-874 osc: prioritize writeback pages

LU-874 osc: prioritize writeback pages

When a lock is being canceled, we should prioritize those covering
pages which have already been submitted by page writeback daemon;
otherwise, this client may be evicted because there is no active IO
for that lock for a long time.

Signed-off-by: Jinshan Xiong <jinshan.xiong@whamcloud.com>
Change-Id: If14eff6361f55d2b2eeb2db7146789dda4c32060
Reviewed-on: http://review.whamcloud.com/1938
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Johann Lombardi <johann@whamcloud.com>
Reviewed-by: Niu Yawei <niu@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>