mbuf: remove aliasing of Tx offloading flags with Rx ones

mbuf: remove aliasing of Tx offloading flags with Rx ones

The reason of redefining the PKT_TX_IPV4 and the PKT_TX_IPV6 is listed below,
It will avoid to send a packet with a bad info:
  - we receive a Ether/IP6/IP4/L4/data packet
  - the driver sets PKT_RX_IPV6_HDR
  - the stack decapsulates IP6
  - the stack sends the packet, it has the PKT_TX_IPV6 flag but it's an IPv4 packet.

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