plugins exts UPDATE ext callback for data node validation

Fixes #1908
diff --git a/src/plugins_exts/schema_mount.c b/src/plugins_exts/schema_mount.c
index 1cfea52..dce83dd 100644
--- a/src/plugins_exts/schema_mount.c
+++ b/src/plugins_exts/schema_mount.c
@@ -850,6 +850,7 @@
         .plugin.compile = &schema_mount_compile,
         .plugin.sprinter = NULL,
         .plugin.free = &schema_mount_free,
+        .plugin.node = NULL,
         .plugin.snode = &schema_mount_snode,
         .plugin.validate = &schema_mount_validate
     },