mbuf: add Tx side tunneling type

mbuf: add Tx side tunneling type

To support tunneling packet offload capabilities on Tx side, PMDs
(e.g., i40e) need to know what kind of tunneling type of this packet.
Instead of analyzing the packet itself, we depend on applications to
correctly set the tunneling type. These flags are defined inside
rte_mbuf.ol_flags.

Signed-off-by: Zhe Tao <zhe.tao@intel.com>
Signed-off-by: Jianfeng Tan <jianfeng.tan@intel.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>