can: isotp: support MSG_TRUNC flag when reading from socket

can: isotp: support MSG_TRUNC flag when reading from socket

BugLink: https://bugs.launchpad.net/bugs/1969110
[ Upstream commit 42bf50a1795a1854d48717b7361dbdbce496b16b ]

When providing the MSG_TRUNC flag via recvmsg() syscall the return value
provides the real length of the packet or datagram, even when it was longer
than the passed buffer.

Fixes: e057dd3fc20f ("can: add ISO 15765-2:2016 transport protocol")
Link: https://github.com/linux-can/can-utils/issues/347#issuecomment-1065932671
Link: https://lore.kernel.org/all/20220316164258.54155-3-socketcan@hartkopp.net
Suggested-by: Derek Will <derekrobertwill@gmail.com>
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Sasha Levin <sashal@kernel.org>
(cherry picked from commit b184a8fa7dd48ae91d7afe7e4a423f29680bc87a)
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>