staging/lustre/llite: to configure max_cached_mb correctly

If there exists MGS conf_param to reduce the memory cache
max_cached_mb it will fail because dt_exp is not initialized
yet.

It should just go ahead and configure it because certainly it
have enough free LRU slot to deduct ccc_lru_left.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-on: http://review.whamcloud.com/11783
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-3676


Reviewed-by: Bobi Jam <bobijam@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>