Only handles packets where the pppoe header length field matches the exact
packet length. Significantly improves rx throughput.
When running NAT traffic through a MediaTek MT7621 devices from a host
behind PPPoE to a host directly connected via ethernet, the TCP throughput
that the device is able to handle improves from ~130 Mbit/s to ~630 Mbit/s,
using fraglist GRO.
Signed-off-by: Felix Fietkau <nbd@nbd.name>