dm stripe: get rid of a Variable Length Array (VLA)

Ideally, we'd like to get rid of all VLAs in the kernel and add -Wvla to
the build args: https://lkml.org/lkml/2018/3/7/621

This one is a simple case, since we don't actually need the VLA at all: we
can just iterate over the stripes twice, once to emit their names, and the
second time to emit status (i.e. trade memory for time). Since the number
of stripes is probably low, this is hopefully not that expensive.
Signed-off-by: Tycho Andersen <tycho@tycho.ws>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-stripe.c

@@ -368,7 +368,6 @@ static void stripe_status(struct dm_target *ti, status_type_t type,
 			  unsigned status_flags, char *result, unsigned maxlen)
 {
 	struct stripe_c *sc = (struct stripe_c *) ti->private;
-	char buffer[sc->stripes + 1];
 	unsigned int sz = 0;
 	unsigned int i;
 
@@ -377,11 +376,12 @@ static void stripe_status(struct dm_target *ti, status_type_t type,
 		DMEMIT("%d ", sc->stripes);
 		for (i = 0; i < sc->stripes; i++)  {
 			DMEMIT("%s ", sc->stripe[i].dev->name);
-			buffer[i] = atomic_read(&(sc->stripe[i].error_count)) ?
-				'D' : 'A';
 		}
-		buffer[i] = '\0';
-		DMEMIT("1 %s", buffer);
+		DMEMIT("1 ");
+		for (i = 0; i < sc->stripes; i++) {
+			DMEMIT("%c", atomic_read(&(sc->stripe[i].error_count)) ?
+			       'D' : 'A');
+		}
 		break;
 
 	case STATUSTYPE_TABLE: