net/compat: Update msg_control_is_user when setting a kernel pointer

cmsghdr_from_user_compat_to_kern() is an unusual case w.r.t. how
the kmsg->msg_control* fields are used. The input struct msghdr
holds a pointer to a user buffer, i.e. ksmg->msg_control_user is
active. However, upon success, a kernel pointer is stored in
kmsg->msg_control. kmsg->msg_control_is_user should therefore be
updated accordingly.

Cc: Christoph Hellwig <hch@lst.de>
Cc: Eric Dumazet <edumazet@google.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/compat.c

@@ -211,6 +211,7 @@ int cmsghdr_from_user_compat_to_kern(struct msghdr *kmsg, struct sock *sk,
 		goto Einval;
 
 	/* Ok, looks like we made it.  Hook it up and return success. */
+	kmsg->msg_control_is_user = false;
 	kmsg->msg_control = kcmsg_base;
 	kmsg->msg_controllen = kcmlen;
 	return 0;