Add a backpointer from boc to busyobj for now.

I don't think this will be necessary at the end state, but it makes
the transformation easier.

bin/varnishd/cache/cache.h

@@ -399,6 +399,7 @@ struct boc {
 	enum busyobj_state_e	state;
 	uint8_t			*vary;
 
+	struct busyobj		*busyobj;
 };
 
 /* Object core structure ---------------------------------------------

bin/varnishd/cache/cache_busyobj.c

@@ -71,6 +71,7 @@ vbo_New(void)
 	INIT_OBJ(bo->boc, BOC_MAGIC);
 	Lck_New(&bo->boc->mtx, lck_busyobj);
 	AZ(pthread_cond_init(&bo->boc->cond, NULL));
+	bo->boc->busyobj = bo;
 	return (bo);
 }