netfilter: ip6tables: Add a .pre_exit hook in all ip6table_foo.c.

Using new helpers ip6t_unregister_table_pre_exit() and
ip6t_unregister_table_exit().

Fixes: b9e69e12

 ("netfilter: xtables: don't hook tables by default")
Signed-off-by: David Wilder <dwilder@us.ibm.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>