[IPSEC]: make sit use the xfrm4_tunnel_register

This patch makes sit use xfrm4_tunnel_register instead of
inet_add_protocol. It solves conflict of sit device with
inter address family IPsec tunnel.

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