btrfs: re-arrange statements in setup_items_for_insert
Rearrange statements calculating the offset of the newly added items so
that the calculation has to be done only once. No functional change.
Reviewed-by: Josef Bacik <josef@toxicpanda.com> Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com> Signed-off-by: Nikolay Borisov <nborisov@suse.com> Reviewed-by: David Sterba <dsterba@suse.com> Signed-off-by: David Sterba <dsterba@suse.com>