LU-1650 obdclass: find the lock by index of subpage

LU-1650 obdclass: find the lock by index of subpage

In check_and_discard_cb(), subpage's index should be used to find if
there exists another lock to cover the same page.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: Id8e1da89ef81fad4c7917360d591fc7bd1309a00
Reviewed-on: http://review.whamcloud.com/4698
Tested-by: Hudson
Reviewed-by: Bobi Jam <bobijam@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Johann Lombardi <johann.lombardi@intel.com>