Btrfs: eliminate a use-after-free in btrfs_balance()

Commit 5af3e8cc introduced a use-after-free at volumes.c:3139: bctl is freed
above in __cancel_balance() in all cases except for balance pause.  Fix this
by moving the offending check a couple statements above, the meaning of the
check is preserved.

Reported-by: Chris Mason <chris.mason@fusionio.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>

fs/btrfs/volumes.c

@@ -3195,6 +3195,11 @@ int btrfs_balance(struct btrfs_balance_control *bctl,
 	mutex_lock(&fs_info->balance_mutex);
 	atomic_dec(&fs_info->balance_running);
 
+	if (bctl->sys.flags & BTRFS_BALANCE_ARGS_CONVERT) {
+		fs_info->num_tolerated_disk_barrier_failures =
+			btrfs_calc_num_tolerated_disk_barrier_failures(fs_info);
+	}
+
 	if (bargs) {
 		memset(bargs, 0, sizeof(*bargs));
 		update_ioctl_balance_args(fs_info, 0, bargs);
@@ -3205,11 +3210,6 @@ int btrfs_balance(struct btrfs_balance_control *bctl,
 		__cancel_balance(fs_info);
 	}
 
-	if (bctl->sys.flags & BTRFS_BALANCE_ARGS_CONVERT) {
-		fs_info->num_tolerated_disk_barrier_failures =
-			btrfs_calc_num_tolerated_disk_barrier_failures(fs_info);
-	}
-
 	wake_up(&fs_info->balance_wait_q);
 
 	return ret;