When destroying the instance, destroy the head rhashtable.

Fixes: 05cd271f ("cls_flower: Support multiple masks per priority")
Reported-by: NVlad Buslov <vladbu@mellanox.com>
Reviewed-by: NRoi Dayan <roid@mellanox.com>
Reviewed-by: NJiri Pirko <jiri@mellanox.com>
Signed-off-by: NPaul Blakey <paulb@mellanox.com>
Reviewed-by: NSimon Horman <simon.horman@netronome.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>