net: Convert lowpan_frags_ops

These pernet_operations register and unregister sysctl.
Also, there is inet_frags_exit_net() called in exit method,
which has to be safe after a5600024 "net: Fix hlist
corruptions in inet_evict_bucket()".

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: David S. Miller <davem@davemloft.net>