malloc: Unmark KASAN redzones if the full allocation size was requested

Consumers that want the full allocation size will typically access the
full buffer, so mark the entire allocation as valid to avoid useless
KASAN reports.

Sponsored by:	The FreeBSD Foundation

sys/kern/kern_malloc.c

@@ -1112,6 +1112,13 @@ malloc_usable_size(const void *addr)
 	else
 		size = malloc_large_size(slab);
 #endif
+
+	/*
+	 * Unmark the redzone to avoid reports from consumers who are
+	 * (presumably) about to use the full allocation size.
+	 */
+	kasan_mark(addr, size, size, 0);
+
 	return (size);
 }