i40e: remove the use of UDP tunnel flag

i40e: remove the use of UDP tunnel flag

The definition of the flag PKT_TX_UDP_TUNNEL_PKT in rte_mbuf.h was:
  TX packet is an UDP tunneled packet. It must be specified when using
  outer checksum offload (PKT_TX_OUTER_IP_CKSUM)

This flag was used to tell the NIC that the offload type is UDP
(I40E_TXD_CTX_UDP_TUNNELING flag). In the datasheet, it says it's
required to specify the tunnel type in the register. However, some tests
(see [1]) showed that it also works without this flag.

Moreover, it is not explained how the hardware use this
information. From a network perspective, this information is useless for
calculating the outer IP checksum as it does not depend on the payload.

Having this flag in the API would force the application to specify the
tunnel type for something that looks only useful for this PMD. It will
limit the number of possible tunnel types (we would need a flag for each
tunnel type) and therefore prevent to support outer IP checksum for
proprietary tunnels.

Finally, if a hardware advertises "I support outer IP checksum", it must
be supported for any payload types.

This has been validated by [2], knowing that the ipip test case was fixed
after this test report [3].

[1] http://dpdk.org/ml/archives/dev/2015-January/011380.html
[2] http://dpdk.org/ml/archives/dev/2015-January/011475.html
[3] http://dpdk.org/ml/archives/dev/2015-January/011610.html

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: Jijiang Liu <jijiang.liu@intel.com>