Rename variable to match option parsing variable name

The data_log_window variable was renamed rgw_data_log_window
in the option parser, but not elsewhere.

This patch fixes it.

Signed-off-by: Christophe Courtaut <christophe.courtaut@gmail.com>
Merge pull request #4 from kri5/wip-datasync

Rename variable to match option parsing variable name

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>