replace numeric ipv6 version with constant
replace numeric ipv6 version with constant

Signed-off-by: Ivan Koveshnikov <64dccc08362effaacf5312029450430dfca124ef@gmail.com>
Reviewed-by: Bogdan Pricope <385103c4eae27e55111df6ab7856eefb0647e14e@enea.com>

OpenFastPath/ofp,TolikH/ofp,OpenFastPath/ofp,TolikH/ofp,TolikH/ofp,OpenFastPath/ofp,OpenFastPath/ofp