dm verity: set DM_TARGET_SINGLETON feature flag

The device-mapper has a flag to mark targets as singleton, which is a
required flag for immutable targets. Without this flag, multiple
dm-verity targets can be added to a mapped device, which has no
practical use cases and will let dm_table_get_immutable_target return
NULL. This patch adds the missing flag, restricting only one
dm-verity target per mapped device.
Signed-off-by: Fan Wu <wufan@linux.microsoft.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

drivers/md/dm-verity-target.c

@@ -1566,7 +1566,7 @@ int dm_verity_get_root_digest(struct dm_target *ti, u8 **root_digest, unsigned i
 
 static struct target_type verity_target = {
 	.name		= "verity",
-	.features	= DM_TARGET_IMMUTABLE,
+	.features	= DM_TARGET_SINGLETON | DM_TARGET_IMMUTABLE,
 	.version	= {1, 10, 0},
 	.module		= THIS_MODULE,
 	.ctr		= verity_ctr,