btrfs: disable qgroups in extent tree v2

Backref lookups are going to be drastically different with extent tree
v2, disable qgroups until we do the work to add this support for extent
tree v2.
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/qgroup.c

@@ -948,6 +948,12 @@ int btrfs_quota_enable(struct btrfs_fs_info *fs_info)
 	 */
 	lockdep_assert_held_write(&fs_info->subvol_sem);
 
+	if (btrfs_fs_incompat(fs_info, EXTENT_TREE_V2)) {
+		btrfs_err(fs_info,
+			  "qgroups are currently unsupported in extent tree v2");
+		return -EINVAL;
+	}
+
 	mutex_lock(&fs_info->qgroup_ioctl_lock);
 	if (fs_info->quota_root)
 		goto out;