Fix: Wrong Enum Variant (#4589)

pageserver/src/pgdatadir_mapping.rs

@@ -887,7 +887,7 @@ impl<'a> DatadirModification<'a> {
         ctx: &RequestContext,
     ) -> Result<(), RelationError> {
         if rel.relnode == 0 {
-            return Err(RelationError::AlreadyExists);
+            return Err(RelationError::InvalidRelnode);
         }
         // It's possible that this is the first rel for this db in this
         // tablespace.  Create the reldir entry for it if so.