s390/diag: Create misc device /dev/diag

Create a misc device /dev/diag to fetch diagnose specific information
from the kernel and provide it to userspace.

Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Sumanth Korikkar <sumanthk@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>

arch/s390/kernel/diag/Makefile

@@ -0,0 +1 @@
+obj-y	:= diag_misc.o

arch/s390/kernel/diag/diag_misc.c

@@ -0,0 +1,45 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Provide diagnose information via misc device /dev/diag.
+ *
+ * Copyright IBM Corp. 2024
+ */
+
+#include <linux/fs.h>
+#include <linux/init.h>
+#include <linux/ioctl.h>
+#include <linux/kernel.h>
+#include <linux/miscdevice.h>
+#include <linux/types.h>
+
+static long diag_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
+{
+	long rc;
+
+	switch (cmd) {
+	default:
+		rc = -ENOIOCTLCMD;
+		break;
+	}
+	return rc;
+}
+
+static const struct file_operations fops = {
+	.owner		= THIS_MODULE,
+	.open		= nonseekable_open,
+	.unlocked_ioctl	= diag_ioctl,
+};
+
+static struct miscdevice diagdev = {
+	.name	= "diag",
+	.minor	= MISC_DYNAMIC_MINOR,
+	.fops	= &fops,
+	.mode	= 0444,
+};
+
+static int diag_init(void)
+{
+	return misc_register(&diagdev);
+}
+
+device_initcall(diag_init);