lib/postprocess: Create OstreeSePolicy more directly

We already called the `workaround_selinux_cross_labeling_recurse()`
in the postprocessing path, there's no need to do it again during
commit.

Just making this change as I was going to do some SePolicy stuff
for the [jigdo work](coreos#1081)
and stumbled again into the ugly mess that is the cross-labeling
hack.

src/libpriv/rpmostree-postprocess.c

@@ -1857,11 +1857,11 @@ rpmostree_commit (int            rootfs_fd,
                   GCancellable  *cancellable,
                   GError       **error)
 {
-  /* hardcode targeted policy for now */
   g_autoptr(OstreeSePolicy) sepolicy = NULL;
   if (enable_selinux)
     {
-      if (!rpmostree_prepare_rootfs_get_sepolicy (rootfs_fd, &sepolicy, cancellable, error))
+      sepolicy = ostree_sepolicy_new_at (rootfs_fd, cancellable, error);
+      if (!sepolicy)
         return FALSE;
     }