Rewrite blocklist functions in order to get progress when

reading large extents and decrease amount of blocklist hook calls.

grub-core/fs/cbfs.c

@@ -289,6 +289,7 @@ grub_cbfsdisk_open (const char *name, grub_disk_t disk)
       return grub_error (GRUB_ERR_UNKNOWN_DEVICE, "not a cbfsdisk");
 
   disk->total_sectors = cbfsdisk_size / GRUB_DISK_SECTOR_SIZE;
+  disk->max_agglomerate = GRUB_DISK_MAX_MAX_AGGLOMERATE;
   disk->id = (unsigned long) "cbfs";
 
   return GRUB_ERR_NONE;