LU-13134 obdclass: use offset instead of cp_linkage

LU-13134 obdclass: use offset instead of cp_linkage

Since we have fixed-size cl_page allocations, we could use
offset array to store every slices pointer for cl_page.

With this patch, we will reduce cl_page size from 392 bytes
to 336 bytes which means we could allocate from 10 to 12 objects.

Change-Id: I323bd589941125bfddf104f53a335d0cfee5c548
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Reviewed-on: https://review.whamcloud.com/37428
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>