examples/ip_fragmentation: fix check of packet type

examples/ip_fragmentation: fix check of packet type

The packet_type in mbuf is not correctly filled by ixgbe 82599 NIC.
To use the ether_type in ethernet header to check packet type is
more reliaber.

Fixes: 3c0184cc0c60 ("examples: replace some offload flags with packet type")
Fixes: ab351fe1c95c ("mbuf: remove packet type from offload flags")
Cc: stable@dpdk.org
Reported-by: Fangfang Wei <fangfangx.wei@intel.com>
Signed-off-by: Wei Dai <wei.dai@intel.com>
Tested-by: Fangfang Wei <fangfangx.wei@intel.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>