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/7194Reviewed-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>

drivers/staging/lustre/lustre/llite/lproc_llite.c

@@ -460,8 +460,8 @@ static ssize_t ll_max_cached_mb_seq_write(struct file *file,
 			break;
 
 		if (!sbi->ll_dt_exp) { /* being initialized */
-			rc = -ENODEV;
-			break;
+			rc = 0;
+			goto out;
 		}
 
 		/* difficult - have to ask OSCs to drop LRU slots. */