ipv6: reorganise struct ipv6_pinfo

ipv6: reorganise struct ipv6_pinfo

Move fields used in tx fast path at the beginning of the structure,
and seldom used ones at the end.

Note that rxopt is also in the first cache line.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20250916160951.541279-5-edumazet@google.com
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>