mm/debug.c: make "migrate_reason_names[]" const char *

Those strings are immutable as well.

Link: http://lkml.kernel.org/r/20181124090508.GB10877@avx2
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/migrate.h

@@ -29,7 +29,7 @@ enum migrate_reason {
 };
 
 /* In mm/debug.c; also keep sync with include/trace/events/migrate.h */
-extern char *migrate_reason_names[MR_TYPES];
+extern const char *migrate_reason_names[MR_TYPES];
 
 static inline struct page *new_page_nodemask(struct page *page,
 				int preferred_nid, nodemask_t *nodemask)

mm/debug.c

@@ -17,7 +17,7 @@
 
 #include "internal.h"
 
-char *migrate_reason_names[MR_TYPES] = {
+const char *migrate_reason_names[MR_TYPES] = {
 	"compaction",
 	"memory_failure",
 	"memory_hotplug",