staging: lustre: 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>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-3676
Reviewed-on: http://review.whamcloud.com/7194


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>