LU-13134 clio: cl_page index compaction

LU-13134 clio: cl_page index compaction

The cpl_index field is not necessary for the vvp layer,
since that's the same as the vm page index, and it has
already been duplicated for the lov layer.

So, moving the osc usage of this to the top, we can save
24 bytes per page (in the slices) by removing this from
the cl_page_slice, at the cost of 8 bytes in the cl_page.
So a net savings of 16 bytes.

After this patch, cl_page could be reduced from 408 to
392 bytes.

Signed-off-by: Patrick Farrell <farr0186@gmail.com>
Change-Id: Ib4fd75f112afb816443799314cd2ef75ff4f4c84
Reviewed-on: https://review.whamcloud.com/37417
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Wang Shilong <wshilong@ddn.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>