When the packet length is smaller than the header length,
the calculated payload length will be overflowed and result
in incorrect reassembly behaviors.
Fixes: 1e4cf4d6d4fb ("gro: cleanup")
Fixes: 9e0b9d2ec0f4 ("gro: support VxLAN GRO")
Cc: stable@dpdk.org
Signed-off-by: Jiayu Hu <jiayu.hu@intel.com>