[lotest] Discard packets arriving on the incorrect network device

[lotest] Discard packets arriving on the incorrect network device

Commit 24bbaf6 ("[lotest] Allow loopback testing on shared networks")
introduced a regression in which loopback testing packets would be
accepted from any network device.  This produces unexpected results,
such as VLAN loopback testing succeeding even when incorrectly using
the underlying trunk device as either transmitter or receiver.

Fix by discarding any loopback testing packets which arrive on a
network device other than the current loopback testing receiver.

Signed-off-by: Michael Brown <mcb30@ipxe.org>