build_backref_tree() uses "goto again;" to implement a breadth-first
search to build backref cache.

This patch will extract most of its work into a wrapper,
handle_one_tree_block(), and use a do {} while() loop to implement the
same thing.
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>