kernel/vm: Default AcquireUnreferencedStoreRef() to B_ERROR.

We shouldn't return B_OK here, because then the page writer will
assume it's acquired a store ref and can write pages from this
cache, when of course it's done nothing of the sort.

src/system/kernel/vm/VMCache.cpp

@@ -1418,7 +1418,7 @@ VMCache::Merge(VMCache* source)
 status_t
 VMCache::AcquireUnreferencedStoreRef()
 {
-	return B_OK;
+	return B_ERROR;
 }