staging: lustre: 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>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-4793
Reviewed-on: http://review.whamcloud.com/10070


Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Bobi Jam <bobijam@gmail.com>
Signed-off-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>