netfilter: xtables: compact table hook functions (2/2)

The calls to ip6t_do_table only show minimal differences, so it seems
like a good cleanup to merge them to a single one too.
Space saving obtained by both patches: 6807725->6807373
("Total" column from `size -A`.)

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>