[INET]: Add missed tunnel64_err handler

The tunnel64_protocol uses the tunnel4_protocol's err_handler and
thus calls the tunnel4_protocol's handlers.

This is not very good, as in case of (icmp) error the wrong error
handlers will be called (e.g. ipip ones instead of sit) and this
won't be noticed at all, because the error is not reported.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>