Fix broken smb_thread_once function

Fix broken smb_thread_once function

- We can't set *ponce=true before running the function because although other
  threads wouldn't re-run the initialization function, they could potentially
  proceed beyond the initialization point while the first thread was still
  running the initialization function. If a second thread gets to an
  SMB_THREAD_ONCE() call while one with the same ponce is running, we need to
  ensure that it enters smb_thread_once() to await the mutex and then recheck
  whether *ponce is set or not.  My original comment about other "once"
  functions possibly being called from within this "once" function is
  irrelevant since those other ones would have their own unique ponce.

Derrell