Fix fstrm_writer_open() to allow multiple opens

[edmonds]

In the iothr implementation, fstrm_writer_open() is called on the writer object during thread startup. However, the caller may have already called fstrm_writer_open() on the writer object before passing it to fstrm_iothr_init(), in which case the second fstrm_writer_open() will fail, and iothr will never write any data.

Modify fstrm_writer_open() to return fstrm_res_success if the writer has already been successfully opened.