vxlan: improve validation of address family configuration

Address families of source and destination addresses must match, and
changelink operations can't change the address family.

In addition, always use the VXLAN_F_IPV6 to check if a VXLAN device uses
IPv4 or IPv6.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>
Signed-off-by: David S. Miller <davem@davemloft.net>