vxlan: Fix bug introduced by commit acbf74a7

Commit acbf74a7 ("vxlan: Refactor vxlan driver to make use of the common UDP tunnel functions." introduced a bug in vxlan_xmit_one()
function, causing it to transmit Vxlan packets without proper
Vxlan header inserted. The change was not needed in the first
place. Revert it.

Reported-by: Tom Herbert <therbert@google.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Andy Zhou <azhou@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>