LU-11221 osd: allow concurrent bulks from pagecache

LU-11221 osd: allow concurrent bulks from pagecache

drop page lock earlier, once IO is complete so that page can be
read by few clients simultanously.

Change-Id: Iee28f578e937744f07f7c5be7eae99e59e625e6e
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/33521
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Li Xi <lixi@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>