datapath: Add support for tun_key to Open vSwitch datapath

datapath: Add support for tun_key to Open vSwitch datapath

This is a first pass at providing a tun_key which can be
used as the basis for flow-based tunnelling. The
tun_key includes and replaces the tun_id in both struct
ovs_skb_cb and struct sw_tun_key.

This patch allows all existing tun_id behaviour to still work. Existing
users of tun_id are redirected to tun_key->tun_id to retain compatibility.
However, when the userspace code is updated to make use of the new
tun_key, the old behaviour will be deprecated and removed.

NOTE: With these changes, the tunneling code no longer assumes input and
output keys are symmetric.  If they are not, PMTUD needs to be disabled
for tunneling to work.

Signed-off-by: Kyle Mestery <kmestery@cisco.com>
Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Reviewed-by: Jesse Gross <jesse@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>