overlay: allow overlaying on top of zfs

OpenZFS works fine as backing fs for overlayfs after:
openzfs/zfs@e015d6c
openzfs/zfs@86db35c
openzfs/zfs@dbf6108

Signed-off-by: Timo Rothenpieler <timo@rothenpieler.org>

drivers/overlay/overlay.go

@@ -356,9 +356,9 @@ func Init(home string, options graphdriver.Options) (graphdriver.Driver, error)
 		if opts.forceMask != nil {
 			return nil, errors.New("'force_mask' is supported only with 'mount_program'")
 		}
-		// check if they are running over btrfs, aufs, zfs, overlay, or ecryptfs
+		// check if they are running over btrfs, aufs, overlay, or ecryptfs
 		switch fsMagic {
-		case graphdriver.FsMagicAufs, graphdriver.FsMagicZfs, graphdriver.FsMagicOverlay, graphdriver.FsMagicEcryptfs:
+		case graphdriver.FsMagicAufs, graphdriver.FsMagicOverlay, graphdriver.FsMagicEcryptfs:
 			return nil, fmt.Errorf("'overlay' is not supported over %s, a mount_program is required: %w", backingFs, graphdriver.ErrIncompatibleFS)
 		}
 		if unshare.IsRootless() && isNetworkFileSystem(fsMagic) {