Fix dbExpand not dividing by slots, resulting in consuming slots times the dictExpand (#12773)

We meant to divide it by the number of slots, otherwise it will do slots
times
dictExpand, bug was introduced in #11695.

src/cluster.h

@@ -80,6 +80,7 @@ int clusterNodeIsMyself(clusterNode *n);
 clusterNode *getMyClusterNode(void);
 char *getMyClusterId(void);
 int getClusterSize(void);
+int getMyClusterSlotCount(void);
 int handleDebugClusterCommand(client *c);
 int clusterNodePending(clusterNode  *node);
 int clusterNodeIsMaster(clusterNode *n);

src/cluster_legacy.c

@@ -5773,6 +5773,10 @@ int getClusterSize(void) {
     return dictSize(server.cluster->nodes);
 }
 
+int getMyClusterSlotCount(void) {
+    return server.cluster->myself->numslots;
+}
+
 char **getClusterNodesList(size_t *numnodes) {
     size_t count = dictSize(server.cluster->nodes);
     char **ids = zmalloc((count+1)*CLUSTER_NAMELEN);

src/db.c

@@ -2196,9 +2196,14 @@ int expireIfNeeded(redisDb *db, robj *key, int flags) {
 int dbExpand(const redisDb *db, uint64_t db_size, dbKeyType keyType, int try_expand) {
     dict *d;
     if (server.cluster_enabled) {
+        /* We don't know exact number of keys that would fall into each slot, but we can
+         * approximate it, assuming even distribution, divide it by the number of slots. */
+        int slots = getMyClusterSlotCount();
+        if (slots == 0) return C_OK;
+        db_size = db_size / slots;
+
         for (int i = 0; i < CLUSTER_SLOTS; i++) {
             if (clusterNodeCoversSlot(getMyClusterNode(), i)) {
-                /* We don't know exact number of keys that would fall into each slot, but we can approximate it, assuming even distribution. */ 
                 if (keyType == DB_MAIN) {
                     d = db->dict[i];
                 } else {