Merge in6_pcbfree() into in_pcbfree() which after the previous
IPsec change in r185366 only differed in two additonal IPv6 lines.
Rather than splattering conditional code everywhere add the v6
check centrally at this single place.

Reviewed by: rwatson (as part of a larger changset)
MFC after: 6 weeks (*)
(*) possibly need to leave a stub wrapper in 7 to keep the symbol.