ipv6: add sysctl_ipv6_flowlabel group

ipv6: add sysctl_ipv6_flowlabel group

Group together following struct netns_sysctl_ipv6 fields:

- flowlabel_consistency
- auto_flowlabels
- flowlabel_state_ranges

After this patch, ip6_make_flowlabel() uses a single cache line to fetch
auto_flowlabels and flowlabel_state_ranges (instead of two before the patch).

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20260115094141.3124990-2-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>