ip6_dst_ops have cache line alignement.

Moving it at beginning of netns_ipv6
removes a 48 byte hole, and shrinks netns_ipv6
from 12 to 11 cache lines.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>