extensions: collapse registration structures

There are no different code paths between IPV4 and IPV6, so
data can be consolidated here.

   text    data     bss     dec     hex filename
 243757   12212    2576  258545   3f1f1 ip6tables-static[before.i586]
 243613    9428    2576  255617   3e681 ip6tables-static[after.i586]
   -144   -2784

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