rxrpc: use udp tunnel APIs instead of open code in rxrpc_open_socket

In rxrpc_open_socket(), now it's using sock_create_kern() and
kernel_bind() to create a udp tunnel socket, and other kernel
APIs to set up it. These code can be replaced with udp tunnel
APIs udp_sock_create() and setup_udp_tunnel_sock(), and it'll
simplify rxrpc_open_socket().

Note that with this patch, the udp tunnel socket will always
bind to a random port if transport is not provided by users,
which is suggested by David Howells, thanks!

Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Reviewed-by: Vadim Fedorenko <vfedorenko@novek.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>