Set be->cooled while holding backends_mtx

Several functions (VBE_Poll and vbe_destroy) tests be->cooled == 0 to
determine which of the two lists backends and cool_backends a specific
instance currently lives on. If the flag is in the process of being
changed, then the wrong list head may be used and will result in strange
bugs.

bin/varnishd/cache/cache_backend.c

@@ -653,9 +653,9 @@ VRT_delete_backend(VRT_CTX, VCL_BACKEND *dp)
 	CAST_OBJ_NOTNULL(be, d->priv, BACKEND_MAGIC);
 	Lck_Lock(&be->mtx);
 	VRT_DisableDirector(be->director);
-	be->cooled = VTIM_real() + 60.;
 	Lck_Unlock(&be->mtx);
 	Lck_Lock(&backends_mtx);
+	be->cooled = VTIM_real() + 60.;
 	VTAILQ_REMOVE(&backends, be, list);
 	VTAILQ_INSERT_TAIL(&cool_backends, be, list);
 	Lck_Unlock(&backends_mtx);