mutex: fix race during locking

If we failed the initial trylock, that does not mean the mutex is locked,
as it could have been unlocked right before we grabbed the wait list lock.

Retry with the wait list lock held, as the unlock path must grab it before
unlocking.

core/mutex.cc

@@ -17,6 +17,11 @@ extern "C" void mutex_lock(mutex_t *mutex)
     w.thread = sched::thread::current();
 
     spin_lock(&mutex->_wait_lock);
+    if (mutex_trylock(mutex)) {
+        // mutex was unlocked just before we grabbed _wait_lock
+        spin_unlock(&mutex->_wait_lock);
+        return;
+    }
     if (!mutex->_wait_list.first) {
         mutex->_wait_list.first = &w;
     } else {