Fix: racy init_done initialization

Fix: racy init_done initialization

If init_done is set while another thread is loading its value, the other
thread can erroneously expect initialization to be complete while the
initialization is still in progress.

Set init_done after initialization with a store-release, and load it
with load-acquire outside of the mutex critical section.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: Idb1345c6dcf3da7d711937bd5072527c0bf1d33a