vxlan: use appropriate family on L3 miss

When sending a L3 miss, the family is set to AF_INET even for IPv6. This
causes userland (eg "ip monitor") to be confused. Ensure we send the
appropriate family in this case. For L2 miss, keep using AF_INET.

Signed-off-by: Vincent Bernat <vincent@bernat.im>
Signed-off-by: David S. Miller <davem@davemloft.net>