nfsd: fix laundromat next-run-time calculation

The laundromat uses two variables to calculate when it should next run,
but one is completely ignored at the end of the run. Merge the two and
rename the variable to be more descriptive of what it does.

Signed-off-by: Jeff Layton <jlayton@primarydata.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>