During build_backref_tree(), if we fail to read a btree node,
we can eventually run into BUG_ON(cache->nr_nodes) that we put
in backref_cache_cleanup(), meaning we have at least one
memory leak.

This frees the backref_node that we's allocated at the very
beginning of build_backref_tree().
Signed-off-by: NLiu Bo <bo.li.liu@oracle.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>