lwtunnel: valid encap attr check should return 0 when lwtunnel is disabled

An error was reported upgrading to 4.9.8:
    root@Typhoon:~# ip route add default table 210 nexthop dev eth0 via 10.68.64.1
    weight 1 nexthop dev eth0 via 10.68.64.2 weight 1
    RTNETLINK answers: Operation not supported

The problem occurs when CONFIG_LWTUNNEL is not enabled and a multipath
route is submitted.

The point of lwtunnel_valid_encap_type_attr is catch modules that
need to be loaded before any references are taken with rntl held. With
CONFIG_LWTUNNEL disabled, there will be no modules to load so the
lwtunnel_valid_encap_type_attr stub should just return 0.

Fixes: 9ed59592 ("lwtunnel: fix autoload of lwt modules")
Reported-by:  <pupilla@libero.it>
Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>