vxlan: simplify vxlan xmit

Existing vxlan xmit function handles two distinct cases.
1. vxlan net device
2. vxlan lwt device.
By seperating initialization these two cases the egress path
looks better.

Signed-off-by: Pravin B Shelar <pshelar@ovn.org>
Acked-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>