alloc_tag: avoid warnings when freeing non-compound "tail" pages

When freeing "tail" pages of a non-compount high-order page, we properly
subtract the allocation tag counters, however later when these pages are
released, alloc_tag_sub() will issue warnings because tags for these pages
are NULL.

This issue was originally anticipated by Vlastimil in his review [1] and
then recently reported by David. Prevent warnings by marking the tags
empty.

Link: https://lkml.kernel.org/r/[e-mail]
Link: https://lore.kernel.org/all/[e-mail]/ [1]
Signed-off-by: Suren Baghdasaryan <[e-mail]>
Suggested-by: David Wang <[e-mail]>
Acked-by: Shakeel Butt <[e-mail]>
Acked-by: Usama Arif <[e-mail]>
Cc: Johannes Weiner <[e-mail]>
Cc: Kent Overstreet <[e-mail]>
Cc: Pasha Tatashin <[e-mail]>
Cc: Sourav Panda <[e-mail]>
Cc: Vlastimil Babka <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

mm/page_alloc.c

···
 		__free_frozen_pages(page, order, fpi_flags);
 	else if (!head) {
 		pgalloc_tag_sub_pages(tag, (1 << order) - 1);
-		while (order-- > 0)
+		while (order-- > 0) {
+			/*
+			 * The "tail" pages of this non-compound high-order
+			 * page will have no code tags, so to avoid warnings
+			 * mark them as empty.
+			 */
+			clear_page_tag_ref(page + (1 << order));
 			__free_frozen_pages(page + (1 << order), order,
 					    fpi_flags);
+		}
 	}
 }