vxlan: reject IPv6 addresses if IPv6 is not configured

When IPv6 address is set without IPv6 configured, the vxlan socket is mostly
treated as an IPv4 one but various lookus in fdb etc. still take the
AF_INET6 into account. This creates incosistencies with weird consequences.

Just reject IPv6 addresses in such case.

Signed-off-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>