btrfs: new test to run btrfs balance and scrub simultaneously

Run btrfs balance and scrub operations simultaneously with fsstress
running in background.

Reviewed-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

common/rc

@@ -2588,6 +2588,15 @@ _btrfs_stress_subvolume()
 	done
 }
 
+# stress btrfs by running scrub in a loop
+_btrfs_stress_scrub()
+{
+	local btrfs_mnt=$1
+	while true; do
+		$BTRFS_UTIL_PROG scrub start -B $btrfs_mnt
+	done
+}
+
 init_rc()
 {
 	if [ "$iam" == new ]