runltp: Do not export empty LTP_BIG_DEV and LTP_BIG_DEV_FS_TYPE

Make runltp export LTP_BIG_DEV and LTP_BIG_DEV_FS_TYPE only when they
have a non-zero value. We do this to make getenv("LTP_BIG_DEV") or
getenv("LTP_BIG_DEV_FS_TYPE") return NULL, if user does not specify a
big block device. Because users usually write such code:

	device = getenv("LTP_BIG_DEV");
	if (device == NULL) {
		tst_brkm(TCONF, NULL,
			 "you must specify a big blockdevice(>1.2G)");
	} else {
		tst_mkfs(NULL, device, "ext3", NULL);
	}

Signed-off-by: Xiaoguang Wang <wangxg.fnst@cn.fujitsu.com>
diff --git a/runltp b/runltp
index 9cb5fe8..ad9b7d1 100755
--- a/runltp
+++ b/runltp
@@ -81,9 +81,7 @@
     export PATH="${PATH}:${LTPROOT}/testcases/bin"
 
     export LTP_DEV=""
-    export LTP_BIG_DEV=""
     export LTP_DEV_FS_TYPE="ext2"
-    export LTP_BIG_DEV_FS_TYPE="ext2"
 
     [ -d "$LTPROOT/testcases/bin" ] ||
     {
@@ -433,7 +431,7 @@
         b) DEVICE=$OPTARG;;
         B) LTP_DEV_FS_TYPE=$OPTARG;;
         z) BIG_DEVICE=$OPTARG;;
-        Z) LTP_BIG_DEV_FS_TYPE=$OPTARG;;
+        Z) BIG_DEVICE_FS_TYPE=$OPTARG;;
         \?) usage;;
         esac
     done
@@ -696,7 +694,12 @@
         echo "Tests which require a big block device are disabled."
         echo "You can specify it with option -z"
     else
-        LTP_BIG_DEV=$BIG_DEVICE
+        export LTP_BIG_DEV=$BIG_DEVICE
+        if [ -z "$BIG_DEVICE_FS_TYPE" ]; then
+            export LTP_BIG_DEV_FS_TYPE="ext2"
+        else
+            export LTP_BIG_DEV_FS_TYPE=$BIG_DEVICE_FS_TYPE
+        fi
     fi
 
     if [ $RUN_REPEATED -gt 1 ]; then # You need to specify at least more than 1 sequential run, else it runs default