staging/lustre/obdclass: limit lu_site hash table size

Allocating a big hash table using the formula for osd
does not really work for clients. We will create new
hash table for each mount on a single client which is
a lot of memory more than expected.

This patch limits the hash table up to 8M which has
524288 entries

Signed-off-by: Li Dongyang <dongyang.li@anu.edu.au>
Reviewed-on: http://review.whamcloud.com/18048
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-7689


Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>