openvswitch: Re-add CONFIG_OPENVSWITCH_VXLAN

openvswitch: Re-add CONFIG_OPENVSWITCH_VXLAN

This readds the config option CONFIG_OPENVSWITCH_VXLAN to avoid a
hard dependency of OVS on VXLAN. It moves the VXLAN config compat
code to vport-vxlan.c and allows compliation as a module.

Orabug: 29755932

Fixes: 614732eaa12d ("openvswitch: Use regular VXLAN net_device device")
Fixes: 2661371ace96 ("openvswitch: fix compilation when vxlan is a module")
Cc: Pravin B Shelar <pshelar@nicira.com>
Cc: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: Thomas Graf <tgraf@suug.ch>
Acked-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit dcc38c033b32b81b88b798f0c0b8453839ac996b)

Reviewed-by: Rama Nichanamatlu <rama.nichanamatlu@oracle.com>
Signed-off-by: Venkat Venkatsubra <venkat.x.venkatsubra@oracle.com>
Signed-off-by: Brian Maly <brian.maly@oracle.com>
Conflicts:
	net/openvswitch/vport-netdev.c

Signed-off-by: Brian Maly <brian.maly@oracle.com>