Fix partition zeroing during prepare

Previously it zeroed area beginning with 0 instead of actual metadata offset
which was leading to non-zeroed metadata when the disk is very small

src/disk_tool/disk_tool_prepare.cpp

@@ -108,6 +108,10 @@ int disk_tool_t::prepare_one(std::map<std::string, std::string> options, int is_
     try
     {
         dsk.parse_config(options);
+        // Set all offsets to 4096 to calculate metadata size with excess
+        dsk.journal_offset = 4096;
+        dsk.meta_offset = 4096;
+        dsk.data_offset = 4096;
         dsk.data_io = dsk.meta_io = dsk.journal_io = (options["io"] == "cached" ? "cached" : "direct");
         dsk.open_data();
         dsk.open_meta();
@@ -498,6 +502,9 @@ int disk_tool_t::get_meta_partition(std::vector<vitastor_dev_info_t> & ssds, std
     {
         blockstore_disk_t dsk;
         dsk.parse_config(options);
+        dsk.journal_offset = 4096;
+        dsk.meta_offset = 4096;
+        dsk.data_offset = 4096;
         dsk.data_io = dsk.meta_io = dsk.journal_io = "cached";
         dsk.open_data();
         dsk.open_meta();