can: gw: indicate and count deleted frames due to misconfiguration

Add a statistic counter to detect deleted frames due to misconfiguration with
a new read-only CGW_DELETED netlink attribute for the CAN gateway.

Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>