carp: Fix packet size checks in carp6_input()

[markj]

Summary

In a couple of places we were checking the packet size against
sizeof(struct ip6_hdr) instead of *offp, but the latter includes
extension headers which we need to skip past to get to the protocol
header. carp_input() handles this properly, copy what it does.

While here, convert carp_input() to use unsigned variables for various
lengths. This is more correct and consistent with carp6_input().

Reported by: Yehyeong Lee (yhlee), [e-mail] (no affiliation)
Fixes: 37115154672f ("carp: support VRRPv3")

[markj]

sys/netinet/ip_carp.c

697

This part of the change is not right, the extension headers are now double-counted in iplen and ntohs(ip6->ip6_plen).

[kp]

Other than Mark's remark I think this looks good.