vxlan: tun_id is 64bit, not 32bit

The tun_id field in struct ip_tunnel_key is __be64, not __be32. We need to
convert the vni to tun_id correctly.

Fixes: 54bfd872 ("vxlan: keep flags and vni in network byte order")
Reported-by: Paolo Abeni <pabeni@redhat.com>
Tested-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Jiri Benc <jbenc@redhat.com>
Acked-by: Thadeu Lima de Souza Cascardo <cascardo@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>