LU-12535 lov: Move page index to top level

LU-12535 lov: Move page index to top level

When doing readahead, we see an amazing amount of time
(~5-8%) just looking up the page index from the lov layer.

In particular, this is more than half the time spent
submitting pages:
         - 14.14% cl_io_submit_rw
            - 13.40% lov_io_submit
               - 8.24% lov_page_index

This requires several indirections, all of which can be
avoided by moving this up to the cl_page struct.

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I99bd7eb4d6556ac89c1aa9aeb4b7afc99774b212
Reviewed-on: https://review.whamcloud.com/35470
Reviewed-by: Wang Shilong <wshilong@ddn.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>