Extract vm_locked_by_ractor_p

This introduces a new method to encapsulate checking whether the current
Ractor owns the vm->ractor.sync lock. This allows us to disable TSan on
it since that operation should be safe, and still get validation of
other uses.

vm_sync.c

@@ -12,7 +12,7 @@ void rb_ractor_sched_barrier_end(rb_vm_t *vm, rb_ractor_t *cr);
 static bool
 vm_locked(rb_vm_t *vm)
 {
-    return vm->ractor.sync.lock_owner == GET_RACTOR();
+    return vm_locked_by_ractor_p(vm, GET_RACTOR());
 }
 
 #if RUBY_DEBUG > 0