[lacp] Detect and ignore erroneously looped back LACP packets

[lacp] Detect and ignore erroneously looped back LACP packets

Some external drivers (observed with the UEFI NII driver provided by
an HPE-branded Mellanox ConnectX-3 Pro) seem to cause LACP packets
transmitted by iPXE to be looped back as received packets.  Since
iPXE's trivial LACP responder will send one response per received
packet, this results in an immediate LACP packet storm.

Detect looped back LACP packets (based on the received LACP actor MAC
address), and refuse to respond to such packets.

Reported-by: Tore Anderson <tore@fud.no>
Tested-by: Tore Anderson <tore@fud.no>
Signed-off-by: Michael Brown <mcb30@ipxe.org>