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>
diff --git a/mm/debug.c b/mm/debug.c
index 72daa4b..0abb987 100644
--- a/mm/debug.c
+++ b/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",