scsi_debug: take sdebug_host_list_lock when changing capacity

All other traversals of the sdebug_host_list take the lock.
Signed-off-by: NEwan D. Milne <emilne@redhat.com>
Acked-by: NDouglas Gilbert <dgilbert@interlog.com>
Signed-off-by: NChristoph Hellwig <hch@lst.de>

drivers/scsi/scsi_debug.c

@@ -4438,6 +4438,7 @@ static ssize_t virtual_gb_store(struct device_driver *ddp, const char *buf,
 			struct sdebug_host_info *sdhp;
 			struct sdebug_dev_info *dp;
 
+			spin_lock(&sdebug_host_list_lock);
 			list_for_each_entry(sdhp, &sdebug_host_list,
 					    host_list) {
 				list_for_each_entry(dp, &sdhp->dev_info_list,
@@ -4446,6 +4447,7 @@ static ssize_t virtual_gb_store(struct device_driver *ddp, const char *buf,
 						dp->uas_bm);
 				}
 			}
+			spin_unlock(&sdebug_host_list_lock);
 		}
 		return count;
 	}