ipv6: udp: leverage scratch area helpers

The commit b65ac446 ("udp: try to avoid 2 cache miss on dequeue")
leveraged the scratched area helpers for UDP v4 but I forgot to
update accordingly the IPv6 code path.

This change extends the scratch area usage to the IPv6 code, synching
the two implementations and giving some performance benefit.
IPv6 is again almost on the same level of IPv4, performance-wide.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>