kernel/do_mount_initrd: move real_root_dev sysctls to its own file

kernel/sysctl.c is a kitchen sink where everyone leaves their dirty
dishes, this makes it very difficult to maintain.

To help with this maintenance let's start by moving sysctls to places
where they actually belong.  The proc sysctl maintainers do not want to
know what sysctl knobs you wish to add for your own piece of code, we
just care about the core logic.

All filesystem syctls now get reviewed by fs folks. This commit
follows the commit of fs, move the real_root_dev sysctl to its own file,
kernel/do_mount_initrd.c.

Signed-off-by: tangmeng <tangmeng@uniontech.com>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>

include/linux/initrd.h

@@ -29,8 +29,6 @@ static inline void wait_for_initramfs(void) {}
 extern phys_addr_t phys_initrd_start;
 extern unsigned long phys_initrd_size;
 
-extern unsigned int real_root_dev;
-
 extern char __initramfs_start[];
 extern unsigned long __initramfs_size;
 

init/do_mounts_initrd.c

@@ -14,12 +14,32 @@
 
 unsigned long initrd_start, initrd_end;
 int initrd_below_start_ok;
-unsigned int real_root_dev;	/* do_proc_dointvec cannot handle kdev_t */
+static unsigned int real_root_dev;	/* do_proc_dointvec cannot handle kdev_t */
 static int __initdata mount_initrd = 1;
 
 phys_addr_t phys_initrd_start __initdata;
 unsigned long phys_initrd_size __initdata;
 
+#ifdef CONFIG_SYSCTL
+static struct ctl_table kern_do_mounts_initrd_table[] = {
+	{
+		.procname       = "real-root-dev",
+		.data           = &real_root_dev,
+		.maxlen         = sizeof(int),
+		.mode           = 0644,
+		.proc_handler   = proc_dointvec,
+	},
+	{ }
+};
+
+static __init int kernel_do_mounts_initrd_sysctls_init(void)
+{
+	register_sysctl_init("kernel", kern_do_mounts_initrd_table);
+	return 0;
+}
+late_initcall(kernel_do_mounts_initrd_sysctls_init);
+#endif /* CONFIG_SYSCTL */
+
 static int __init no_initrd(char *str)
 {
 	mount_initrd = 0;

kernel/sysctl.c

@@ -1697,15 +1697,6 @@ static struct ctl_table kern_table[] = {
 		.mode		= 0644,
 		.proc_handler	= sysctl_latencytop,
 	},
-#endif
-#ifdef CONFIG_BLK_DEV_INITRD
-	{
-		.procname	= "real-root-dev",
-		.data		= &real_root_dev,
-		.maxlen		= sizeof(int),
-		.mode		= 0644,
-		.proc_handler	= proc_dointvec,
-	},
 #endif
 	{
 		.procname	= "print-fatal-signals",