vxlan: do not use fdb in metadata mode

In metadata mode, the vxlan interface is not supposed to use the fdb control
plane but an external one (openvswitch or static routes). With the current
code, packets may leak into the fdb handling code which usually causes them
to be dropped anyway but may have strange side effects.

Just drop the packets directly when in metadata mode if the destination data
are not correctly provided on egress.

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