Condvar: Move unlock of user mutex

Move the unlocking of the user's mutex in condvar_wait() a bit earlier,
while we still hold the condvar's internal mutex.

This does not change correctness, but it is needed for the wait morphing
protocol, where we assume that once condvar_wake() finds this thread's
wait_record (which can happen as soon as we release the internal mutex),
we are no longer holding the user mutex.

core/condvar.cc

@@ -26,9 +26,9 @@ int condvar_wait(condvar_t *condvar, mutex_t* user_mutex, sched::timer* tmr)
         condvar->waiters_fifo.newest->next = ≀
     }
     condvar->waiters_fifo.newest = ≀
+    mutex_unlock(user_mutex);
     mutex_unlock(&condvar->m);
 
-    mutex_unlock(user_mutex);
     // Wait until either the timer expires or condition variable signaled
     wr.wait(tmr);
     if (!wr.woken()) {