memory tier: make memory_tier_subsys const

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

Link: https://lkml.kernel.org/r/20240204-bus_cleanup-mm-v1-1-00f49286f164@marliere.netSigned-off-by: Ricardo B. Marliere <ricardo@marliere.net>
Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/memory-tiers.c

@@ -39,7 +39,7 @@ static LIST_HEAD(memory_tiers);
 static struct node_memory_type_map node_memory_types[MAX_NUMNODES];
 struct memory_dev_type *default_dram_type;
 
-static struct bus_type memory_tier_subsys = {
+static const struct bus_type memory_tier_subsys = {
 	.name = "memory_tiering",
 	.dev_name = "memory_tier",
 };