inet: frags: refactor ipfrag_init()

We need to call inet_frags_init() before register_pernet_subsys(),
as a prereq for following patch ("inet: frags: use rhashtables for reassembly units")

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit 483a6e4fa055123142d8956866fe2aa9c98d546d)
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>