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: NDavid Ahern <dsa@cumulusnetworks.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/net/lwtunnel.h

@@ -178,7 +178,10 @@ static inline int lwtunnel_valid_encap_type(u16 encap_type)
 }
 static inline int lwtunnel_valid_encap_type_attr(struct nlattr *attr, int len)
 {
-	return -EOPNOTSUPP;
+	/* return 0 since we are not walking attr looking for
+	 * RTA_ENCAP_TYPE attribute on nexthops.
+	 */
+	return 0;
 }
 
 static inline int lwtunnel_build_state(struct net_device *dev, u16 encap_type,