mm: check __highest_present_section_nr directly in memory_dev_init()

__highest_present_section_nr is a more strict boundary than
NR_MEM_SECTIONS.  So checking __highest_present_section_nr directly is
enough.

Link: http://lkml.kernel.org/r/20180330032044.21647-1-richard.weiyang@gmail.com
Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/base/memory.c

@@ -837,11 +837,8 @@ int __init memory_dev_init(void)
 	 * during boot and have been initialized
 	 */
 	mutex_lock(&mem_sysfs_mutex);
-	for (i = 0; i < NR_MEM_SECTIONS; i += sections_per_block) {
-		/* Don't iterate over sections we know are !present: */
-		if (i > __highest_present_section_nr)
-			break;
-
+	for (i = 0; i <= __highest_present_section_nr;
+		i += sections_per_block) {
 		err = add_memory_block(i);
 		if (!ret)
 			ret = err;