mm/damon/vaddr: add a comment for 'default' case in damon_va_apply_scheme()

The switch case 'DAMOS_STAT' and switch case 'default' have same return
value in damon_va_apply_scheme(), and the 'default' case is for DAMOS
actions that not supported by 'vaddr'.  It might make sense to add a
comment here.

[akpm@linux-foundation.org: fx comment grammar]
Link: https://lkml.kernel.org/r/1662606797-23534-1-git-send-email-kaixuxia@tencent.com
Signed-off-by: Kaixu Xia <kaixuxia@tencent.com>
Reviewed-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/damon/vaddr.c

@@ -658,6 +658,9 @@ static unsigned long damon_va_apply_scheme(struct damon_ctx *ctx,
 	case DAMOS_STAT:
 		return 0;
 	default:
+		/*
+		 * DAMOS actions that are not yet supported by 'vaddr'.
+		 */
 		return 0;
 	}