H5VLattr_optional

herr_t H5VLattr_optional(void *  attr,  hid_t connector_id,  H5VL_optional_args_t args,
       hid_t dxpl_id,  void * *  req)

Performs an optional operation on an attribute through the VOL.

`attr`	Attribute object
`connector_id`	VOL connector identifier
`args`	Optional operation arguments
`dxpl_id`	Data transfer property list
`req`	Async request object (can be null)

Non-negative on success, negative on failure.