btrfs: extent-tree: cleanup one-shot usage of @blocksize in do_walk_down

@blocksize variable in do_walk_down() is only used once, really no need
to declare it.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>