Fix mutex_free so it doesn't call into Ruby code during GC

While freeing a locked mutex, if there is a waiter for the
mutex it could call `rb_fiber_scheduler_unblock` which calls
into Ruby code.

The function `rb_mutex_unlock_th` was a footgun because it was used
during normal Ruby code and also during GC. The fix is to not call this
function during GC and simply remove the mutex from th->keeping_mutexes.

Author: luke-gruber

thread_sync.c

@@ -148,13 +148,14 @@ mutex_locked_p(rb_mutex_t *mutex)
     return mutex->ec_serial != 0;
 }
 
+static void thread_mutex_remove(rb_thread_t *thread, rb_mutex_t *mutex);
+
 static void
 mutex_free(void *ptr)
 {
     rb_mutex_t *mutex = ptr;
     if (mutex_locked_p(mutex)) {
-        const char *err = rb_mutex_unlock_th(mutex, mutex->th, 0, true);
-        if (err) rb_bug("%s", err);
+        thread_mutex_remove(mutex->th, mutex);
     }
     ruby_xfree(ptr);
 }