Fix fco on LRU twice

2d468a6f gave the clue:

When fellow_cache_obj_lru_touch() raced fellow_cache_lru_work(),
we would put the fco's fcs on the lru list twice: once from
_touch and then from _lru_work().

Fixes #25

src/fellow_cache.c

@@ -3427,6 +3427,10 @@ fellow_cache_obj_lru_touch(struct fellow_cache_obj *fco)
 	if (! fcs->fcs_onlru)
 		return (0);
 
+	// mutate should be failing, so it will do the lru move
+	if (fcs->fco_lru_mutate)
+		return (1);
+
 	assert(fellow_cache_shouldlru(fcs->state, fco->oc, fcs->refcnt));
 
 	r = pthread_mutex_trylock(&lru->lru_mtx);
@@ -3434,9 +3438,10 @@ fellow_cache_obj_lru_touch(struct fellow_cache_obj *fco)
 		assert(r == EBUSY);
 		return (0);
 	}
-	AZ(fcs->fco_lru_mutate);
-	VTAILQ_REMOVE(&lru->lru_head, fcs, lru_list);
-	VTAILQ_INSERT_TAIL(&lru->lru_head, fcs, lru_list);
+	if (! fcs->fco_lru_mutate) {
+		VTAILQ_REMOVE(&lru->lru_head, fcs, lru_list);
+		VTAILQ_INSERT_TAIL(&lru->lru_head, fcs, lru_list);
+	}
 	//lint -e{455} flexelint does not grok trylock
 	AZ(pthread_mutex_unlock(&lru->lru_mtx));