netfilter: ipv6: nf_defrag: no longer reference init_net in nf_ct_frag6_sysctl_table

(struct net *)->nf_frag.fqdir will soon be a pointer, so make
sure nf_ct_frag6_sysctl_table[] does not reference init_net.

nf_ct_frag6_sysctl_register() can perform the needed initialization
for all netns.

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