[IPSEC]: changing API of xfrm6_tunnel_register

This patch changes xfrm6_tunnel register and deregister
interface to prepare for solving the conflict of device
tunnels with inter address family IPsec tunnel.
There is no device which conflicts with IPv4 over IPv6
IPsec tunnel.

Signed-off-by: Kazunori MIYAZAWA <miyazawa@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>