ipv6: no longer reference init_net in ip6_frags_ns_ctl_table[]

(struct net *)->ipv6.fqdir will soon be a pointer, so make
sure ip6_frags_ns_ctl_table[] does not reference init_net.

ip6_frags_ns_ctl_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>