Btrfs: fix orphan backref nodes

If the root node of a fs/file tree is in the block group that is
being relocated, but the others are not in the other block groups.
when we create a snapshot for this tree between the relocation tree
creation ends and ->create_reloc_tree is set to 0, Btrfs will create
some backref nodes that are the lowest nodes of the backrefs cache.
But we forget to add them into ->leaves list of the backref cache
and deal with them, and at last, they will triggered BUG_ON().

  kernel BUG at fs/btrfs/relocation.c:239!

This patch fixes it by adding them into ->leaves list of backref cache.
Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/relocation.c

@@ -1174,6 +1174,8 @@ static int clone_backref_node(struct btrfs_trans_handle *trans,
 			list_add_tail(&new_edge->list[UPPER],
 				      &new_node->lower);
 		}
+	} else {
+		list_add_tail(&new_node->lower, &cache->leaves);
 	}
 
 	rb_node = tree_insert(&cache->rb_root, new_node->bytenr,