libroot: Downgrade lock assertion into a syslog print.

It seems there has been a bug lurking for years that this exposed.
Rather than inconvenience users further, as it's not especially
hard to trigger, let's down-grade it into a message until it can
be properly investigated and fixed.

Related to #18451.

src/system/libroot/os/locks/mutex.cpp

@@ -90,6 +90,12 @@ __mutex_unlock(mutex *lock)
 		_kern_mutex_unblock(&lock->lock, 0);
 	}
 
-	if ((oldValue & B_USER_MUTEX_LOCKED) == 0)
+	if ((oldValue & B_USER_MUTEX_LOCKED) == 0) {
+#if 0
 		debugger("mutex was not actually locked!");
+#else
+		// The above happens too often at present (see bug #18451).
+		_kern_debug_output("libroot __mutex_unlock: mutex was not actually locked!\n");
+#endif
+	}
 }