ipv6: add ipv6_devconf_read_txrx cacheline_group

IPv6 TX and RX fast path use the following fields:

- disable_ipv6
- hop_limit
- mtu6
- forwarding
- disable_policy
- proxy_ndp

Place them in a group to increase data locality.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>