[lacp] Ignore (and do not echo) trailing padding on received packets

[lacp] Ignore (and do not echo) trailing padding on received packets

The LACP responder reuses the received I/O buffer to construct the
response LACP (or marker) packet.  Any received padding will therefore
be unintentionally included within the response.

Truncate the received I/O buffer to the expected length (which is
already defined in a way to allow for future protocol expansion)
before reusing it to construct the response.

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