btrfs: reloc: open code read_fs_root() for handle_indirect_tree_backref()

The backref code is going to be moved to backref.c, and read_fs_root()
is just a simple wrapper, open-code it to prepare to the incoming code
move.
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/relocation.c

@@ -473,12 +473,16 @@ static int handle_indirect_tree_backref(struct btrfs_backref_cache *cache,
 	struct btrfs_backref_edge *edge;
 	struct extent_buffer *eb;
 	struct btrfs_root *root;
+	struct btrfs_key root_key;
 	struct rb_node *rb_node;
 	int level;
 	bool need_check = true;
 	int ret;
 
-	root = read_fs_root(fs_info, ref_key->offset);
+	root_key.objectid = ref_key->offset;
+	root_key.type = BTRFS_ROOT_ITEM_KEY;
+	root_key.offset = (u64)-1;
+	root = btrfs_get_fs_root(fs_info, &root_key, false);
 	if (IS_ERR(root))
 		return PTR_ERR(root);
 	if (!test_bit(BTRFS_ROOT_REF_COWS, &root->state))