Assert that fco LRU state is not changed during mutation

src/fellow_cache.c

@@ -1167,6 +1167,7 @@ fellow_cache_lru_chg(struct fellow_lru_chgbatch *lcb,
 	assert(add ==
 	    fellow_cache_shouldlru(fcs->state, fcs->fco->oc, fcs->refcnt));
 #endif
+	AZ(fcs->fco_lru_mutate);
 	assert(fcs->fcs_onlru == (unsigned)!add);
 	fcs->fcs_onlru = add;
 
@@ -3462,6 +3463,7 @@ fellow_cache_obj_lru_touch(struct fellow_cache_obj *fco)
 		assert(r == EBUSY);
 		return (0);
 	}
+	AZ(fcs->fco_lru_mutate);
 	BUG25_CHECK(fcs);
 	VTAILQ_REMOVE(&lru->lru_head, fcs, lru_list);
 	BUG25_CHECK(fcs);