vxlan: use preferred address family when neither group or remote is specified

vxlan: use preferred address family when neither group or remote is specified

When neither group or remote is specified (or if they are specified with
the any address), nothing is sent to the kernel. In this case, the
kernel defaults to IPv4. This makes impossible to use IPv6 with
unspecified unicast remote ("bridge fdb add" will return
EAFNOTSUPPORT).

If the user specifies a preferred address family (eg, "ip -6 link add"),
then send either IFLA_VXLAN_GROUP or IFLA_VXLAN_GROUP6 to enforce the
use of the appropriate family.

Signed-off-by: Vincent Bernat <vincent@bernat.im>