netfilter: x_tables: add and use xt_unregister_table_pre_exit

netfilter: x_tables: add and use xt_unregister_table_pre_exit

Remove the copypasted variants of _pre_exit and add one single
function in the xtables core.  ebtables is not compatible with
x_tables and therefore unchanged.

This is a preparation patch to reduce noise in the followup
bug fixes.

Reviewed-by: Tristan Madani <tristan@talencesecurity.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>