rxrpc: Fix dependency on IPv6 in udp tunnel config

As udp_port_cfg struct changes its members with dependency on IPv6
configuration, the code in rxrpc should also check for IPv6.

Fixes: 1a9b86c9 ("rxrpc: use udp tunnel APIs instead of open code in rxrpc_open_socket")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Vadim Fedorenko <vfedorenko@novek.ru>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>