Btrfs: add missing error check to find_parent_nodes

Signed-off-by: Filipe David Borba Manana <fdmanana@gmail.com>
Reviewed-by: Jan Schmidt <list.btrfs@jan-o-sch.net>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>