LU-3676 llite: error setting max_cache_mb at mount time

LU-3676 llite: error setting max_cache_mb at mount time

The root cause is that when max_cache_mb conf parameter is applied,
the client isn't connected to the OST yet so that sbi->ll_dt_exp is
NULL. However, it's not necessary to shrink the cache memory in this
case so success should be returned.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: Ib12800812b635207bf4001e2272aadb0ce1e0d6f
Reviewed-on: http://review.whamcloud.com/7194
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Bobi Jam <bobijam@gmail.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>