LU-4793 clio: Reduce memory overhead of per-page allocation

LU-4793 clio: Reduce memory overhead of per-page allocation

A page in clio used to occupy 584 bytes, which will use size-1024
slab cache. This patch reduces the per-page overhead to 512 bytes
so it can use size-512 instead.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: I54d7ed7b64d5987ddaafbf16edafa5cdf491b0a1
Reviewed-on: http://review.whamcloud.com/10070
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Bobi Jam <bobijam@gmail.com>