feat(storage): add mkdir to FileSystem trait with stubs

crates/servers/storage-server/src/fs/fat16.rs

@@ -368,6 +368,8 @@ impl<B: BlockDevice> FileSystem for Fat16<B> {
     fn file_size(&mut self, handle: FileHandle) -> Option<usize> {
         self.open_files[handle.raw() as usize].as_ref().map(|f| f.size)
     }
+
+    fn mkdir(&mut self, _path: &[u8]) -> bool { false }
 }
 
 fn to_fat_name(name: &[u8]) -> [u8; 11] {