Unbreak ip_carp with WITHOUT_INET6 enabled by conditionalizing all IPv6

Unbreak ip_carp with WITHOUT_INET6 enabled by conditionalizing all IPv6
structs under the INET6 #ifdef. Similarly (even though it doesn't seem
to affect the build), conditionalize all IPv4 structs under the INET
#ifdef

This also unbreaks the LINT-NOINET6 tinderbox target on amd64; I have not
verified other MACHINE/TARGET pairs (e.g. armv6/arm).

MFC after:	2 weeks
X-MFC with:	r310847
Pointyhat to:	jpaetzel
Reported by:	O. Hartmann <o.hartmann@walstatt.org>