MDEV-22765 i_s_fts_index_cache_fill_one_index() is not protect by the lock

- i_s_fts_index_cache_fill() should take shared lock of fts cache
before accessing index cache to avoid reading stale data.

storage/xtradb/handler/i_s.cc

@@ -3367,6 +3367,8 @@ no_fts:
 	conv_str.f_len = sizeof word;
 	conv_str.f_str = word;
 
+	rw_lock_s_lock(&cache->lock);
+
 	for (ulint i = 0; i < ib_vector_size(cache->indexes); i++) {
 		fts_index_cache_t*      index_cache;
 
@@ -3377,6 +3379,7 @@ no_fts:
 				 index_cache, thd, &conv_str, tables));
 	}
 
+	rw_lock_s_unlock(&cache->lock);
 	dict_table_close(user_table, FALSE, FALSE);
 	rw_lock_s_unlock(&dict_operation_lock);