can: isotp: restore accidentally removed MSG_PEEK feature

[ Upstream commit e382fea ]

In commit 42bf50a ("can: isotp: support MSG_TRUNC flag when
reading from socket") a new check for recvmsg flags has been
introduced that only checked for the flags that are handled in
isotp_recvmsg() itself.

This accidentally removed the MSG_PEEK feature flag which is processed
later in the call chain in __skb_try_recv_from_queue().

Add MSG_PEEK to the set of valid flags to restore the feature.

Fixes: 42bf50a ("can: isotp: support MSG_TRUNC flag when reading from socket")
Link: linux-can/can-utils#347 (comment)
Link: https://lore.kernel.org/all/20220328113611.3691-1-socketcan@hartkopp.net
Reported-by: Derek Will <derekrobertwill@gmail.com>
Suggested-by: Derek Will <derekrobertwill@gmail.com>
Tested-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>

Author: hartkopp

net/can/isotp.c

@@ -1007,7 +1007,7 @@ static int isotp_recvmsg(struct socket *sock, struct msghdr *msg, size_t size,
 	int noblock = flags & MSG_DONTWAIT;
 	int ret = 0;
 
-	if (flags & ~(MSG_DONTWAIT | MSG_TRUNC))
+	if (flags & ~(MSG_DONTWAIT | MSG_TRUNC | MSG_PEEK))
 		return -EINVAL;
 
 	if (!so->bound)