libostree: write selinux xattr when on non-selinux systems

Currently when writing data for selinux systems on a non-selinux
system there will be no labels. This is because
`ostree_sepolicy_setfscreatecon()` just returns TRUE on non-selinux
systems and xattr writing for `security.seliux` is filtered out.

This patches uses the suggestion of Colin Walters (thanks!) from
https://github.com/ostreedev/ostree/issues/2804 and detects if
the host has selinux enabled and if not just skips filtering the
xattrs for selinux.

src/libostree/ostree-repo-checkout.c

@@ -195,7 +195,7 @@ create_file_copy_from_input_at (OstreeRepo *repo, OstreeRepoCheckoutAtOptions *o
   g_autoptr (GVariant) modified_xattrs = NULL;
 
   /* If we're doing SELinux labeling, prepare it */
-  if (sepolicy_enabled)
+  if (sepolicy_enabled && ostree_sepolicy_host_enabled (options->sepolicy))
     {
       /* If doing sepolicy path-based labeling, we don't want to set the
        * security.selinux attr via the generic xattr paths in either the symlink
@@ -1045,7 +1045,7 @@ checkout_tree_at_recurse (OstreeRepo *self, OstreeRepoCheckoutAtOptions *options
     };
 
     /* If we're doing SELinux labeling, prepare it */
-    if (sepolicy_enabled)
+    if (sepolicy_enabled && ostree_sepolicy_host_enabled (options->sepolicy))
       {
         /* We'll set the xattr via setfscreatecon(), so don't do it via generic xattrs below. */
         modified_xattrs = _ostree_filter_selinux_xattr (xattrs);

src/libostree/ostree-sepolicy.c

@@ -753,3 +753,19 @@ _ostree_filter_selinux_xattr (GVariant *xattrs)
     return NULL;
   return g_variant_ref_sink (g_variant_builder_end (&builder));
 }
+
+/**
+ * ostree_sepolicy_host_enabled:
+ * @self: Policy
+ *
+ * Return if the host has selinux enabled
+ */
+gboolean
+ostree_sepolicy_host_enabled (OstreeSePolicy *self)
+{
+#ifdef HAVE_SELINUX
+  return cached_is_selinux_enabled ();
+#else
+  return FALSE;
+#endif
+}

src/libostree/ostree-sepolicy.h

@@ -77,4 +77,7 @@ void ostree_sepolicy_fscreatecon_cleanup (void **unused);
 #define ostree_cleanup_sepolicy_fscreatecon \
   __attribute__ ((cleanup (ostree_sepolicy_fscreatecon_cleanup)))
 
+_OSTREE_PUBLIC
+gboolean ostree_sepolicy_host_enabled (OstreeSePolicy *self);
+
 G_END_DECLS