[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>

src/net/eth_slow.c

@@ -286,6 +286,9 @@ static int eth_slow_rx ( struct io_buffer *iobuf,
 		return -EINVAL;
 	}
 
+	/* Strip any trailing padding */
+	iob_unput ( iobuf, ( sizeof ( *eth_slow ) - iob_len ( iobuf ) ) );
+
 	/* Handle according to subtype */
 	switch ( eth_slow->header.subtype ) {
 	case ETH_SLOW_SUBTYPE_LACP: