Fix use of wrong 'i' variable name

Fix use of wrong 'i' variable name

pylint complained:
tarfile.py:3797:38: E0602: Undefined variable 'i' (undefined-variable)

Code was introduced like this in

commit 27ee4dd4df48340541317123f5df348056c235ca
Author:     Philipp Gesang <philipp.gesang@intra2net.com>
AuthorDate: Tue Aug 29 12:00:54 2017 +0200

    implement volume handling for rescue mode

    When reconstructing the index, traverse backup volumes and set
    the “volume” member on the objects appropriately.

-> I guess 'i' was renamed to nvol for better readability.