LU-3676 llite: to configure max_cached_mb correctly

LU-3676 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.

This patch is back-ported from the following one:
Lustre-commit: b0135d72b783eed4d9b86532be6244ebc57b1f39
Lustre-change: http://review.whamcloud.com/11783

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: If837f178592dd487775ddcb15115964fba6429bf
Reviewed-on: http://review.whamcloud.com/12924
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>