geneve: Add support to collect tunnel metadata.

Following patch create new tunnel flag which enable
tunnel metadata collection on given device. These devices
can be used by tunnel metadata based routing or by OVS.
Geneve Consolidation patch get rid of collect_md_tun to
simplify tunnel lookup further.

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Reviewed-by: Jesse Gross <jesse@nicira.com>
Acked-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>