driver core: cpu: make cpu_subsys const

Now that the driver core can properly handle constant struct bus_type,
move the cpu_subsys variable to be a constant structure as well, placing
it into read-only memory which can not be modified at runtime.

Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Link: https://lore.kernel.org/r/2024010548-crane-snooze-a871@gregkhSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/base/cpu.c

@@ -366,7 +366,7 @@ static int cpu_uevent(const struct device *dev, struct kobj_uevent_env *env)
 }
 #endif
 
-struct bus_type cpu_subsys = {
+const struct bus_type cpu_subsys = {
 	.name = "cpu",
 	.dev_name = "cpu",
 	.match = cpu_subsys_match,

include/linux/cpu.h

@@ -128,7 +128,7 @@ static inline void cpu_maps_update_done(void)
 static inline int add_cpu(unsigned int cpu) { return 0;}
 
 #endif /* CONFIG_SMP */
-extern struct bus_type cpu_subsys;
+extern const struct bus_type cpu_subsys;
 
 extern int lockdep_is_cpus_held(void);