vxlan: treat vni in metadata based tunnels consistently

For metadata based tunnels, VNI is ignored when doing vxlan device lookups
(because such tunnel receives all VNIs). However, this was not honored by
vxlan_xmit_one when doing encapsulation bypass. Move the check for metadata
based tunnel to the common place where it belongs.

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