ipv4: remove the useless argument from ip_tunnel_hash()

Since commit c5441932("GRE: Refactor GRE tunneling code")
introduced function ip_tunnel_hash(), the argument itn is no
longer in use, so remove it.

Signed-off-by: Duan Jiong <duanj.fnst@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>