af_key: fix netns ops ordering on module load/unload

1. After sock_register() returns, it's possible to create sockets,
   even if module still not initialized fully (blame generic module code
   for that!)
2. Consequently, pfkey_create() can be called with pfkey_net_id still not
   initialized which will BUG_ON in net_generic():
	kernel BUG at include/net/netns/generic.h:43!
3. During netns shutdown, netns ops should be unregistered after
   key manager unregistered because key manager calls can be triggered
   from xfrm_user module:

   	general protection fault: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
	pfkey_broadcast+0x111/0x210 [af_key]
	pfkey_send_notify+0x16a/0x300 [af_key]
	km_state_notify+0x41/0x70
	xfrm_flush_sa+0x75/0x90 [xfrm_user]
4. Unregister netns ops after socket ops just in case and for symmetry.

Reported by Luca Tettamanti.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Tested-by: Luca Tettamanti <kronos.it@gmail.com>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>