btrfs: use btrfs_for_each_slot in is_ancestor

This function can be simplified by refactoring to use the new iterator
macro.  No functional changes.

Signed-off-by: Marcos Paulo de Souza <mpdesouza@suse.com>
Signed-off-by: Gabriel Niebler <gniebler@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>