vxlan: fix tos value before xmit

Before ip_tunnel_ecn_encap() and udp_tunnel_xmit_skb() we should filter
tos value by RT_TOS() instead of using config tos directly.

vxlan_get_route() would filter the tos to fl4.flowi4_tos but we didn't
return it back, as geneve_get_v4_rt() did. So we have to use RT_TOS()
directly in function ip_tunnel_ecn_encap().

Fixes: 206aaafc ("VXLAN: Use IP Tunnels tunnel ENC encap API")
Fixes: 1400615d ("vxlan: allow setting ipv6 traffic class")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>