ieee820154: 6lowpan: no longer reference init_net in lowpan_frags_ns_ctl_table

(struct net *)->ieee802154_lowpan.fqdir will soon be a pointer, so make
sure lowpan_frags_ns_ctl_table[] does not reference init_net.

lowpan_frags_ns_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>