LU-948 clio: add a callback to cl_page_gang_lookup()

LU-948 clio: add a callback to cl_page_gang_lookup()

Add a callback to cl_page_gang_lookup() so that it will be easier to
fix this issue and be helpful for new IO engine.

If a read lock is being canceled, we used to grab page lock and then
check if they are covered by another lock, otherwise they will be
discarded. This is unnecessary because we can do this w/o grabbing
page lock.

With the above fix, when a read-ahead page is in IO during recovery,
and one of covering locks is being canceled by early cancel for
recovery, it will detect that this page is being covered by another
one, and then this page will be skipped w/o trying to grab page lock.

Signed-off-by: Jinshan Xiong <jinshan.xiong@whamcloud.com>
Change-Id: I22a3ea0790f5c0e01c12c29208b6d60c38058f12
Reviewed-on: http://review.whamcloud.com/1955
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>