"inpcb", and this struct inpcb* and struct inp6cb* are the same type.
On NetBSD they are different types, so we must change the types of
formal argument in IPv6-specific functions from "struct inpcb *" to
"struct in6pcb*".
The code didn't compile on NetBSD beforehand, if both FAST_IPSEC + INET6
were configured. This fix will cause even more short-term breakage for
that case, but its a step in the right direction: it shows up what
still needs to be fixed.
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