Address code review feedback

src/libraries/Common/src/Interop/Unix/System.Native/Interop.MkDir.cs

@@ -14,9 +14,8 @@ internal static partial class Sys
 
         internal static int MkDir(ReadOnlySpan<char> path, int mode)
         {
-            var converter = new ValueUtf8Converter(stackalloc byte[DefaultPathBufferSize]);
+            using ValueUtf8Converter converter = new(stackalloc byte[DefaultPathBufferSize]);
             int result = MkDir(ref MemoryMarshal.GetReference(converter.ConvertAndTerminateString(path)), mode);
-            converter.Dispose();
             return result;
         }
     }

src/libraries/System.Private.CoreLib/src/System/IO/FileSystem.Unix.cs

@@ -300,26 +300,19 @@ public static void CreateDirectory(string fullPath)
             }
             else if (errorInfo.Error == Interop.Error.ENOENT) // Some parts of the path don't exist yet.
             {
-                // Try create parents bottom to top and track those that could not
-                // be created due to missing parents. Then create them top to bottom.
-                ValueListBuilder<int> stackDir = new(stackalloc int[32]); // 32 arbitrarily chosen
-                try
-                {
-                    CreateParentsAndDirectory(fullPath, ref stackDir);
-                }
-                finally
-                {
-                    stackDir.Dispose();
-                }
+                CreateParentsAndDirectory(fullPath);
             }
             else
             {
                 throw Interop.GetExceptionForIoErrno(errorInfo, fullPath, isDirectory: true);
             }
         }
 
-        private static void CreateParentsAndDirectory(string fullPath, ref ValueListBuilder<int> stackDir)
+        private static void CreateParentsAndDirectory(string fullPath)
         {
+            // Try create parents bottom to top and track those that could not
+            // be created due to missing parents. Then create them top to bottom.
+            using ValueListBuilder<int> stackDir = new(stackalloc int[32]); // 32 arbitrarily chosen
             stackDir.Append(fullPath.Length);
 
             int i = fullPath.Length - 1;