vxlan: Expose vxlan_xmit_one()

Given a packet and a remote destination, the function will take care of
encapsulating the packet and transmitting it to the destination.

Expose it so that it could be used in subsequent patches by the MDB code
to transmit a packet to the remote destination(s) stored in the MDB
entry.

It will allow us to keep the MDB code self-contained, not exposing its
data structures to the rest of the VXLAN driver.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: David S. Miller <davem@davemloft.net>