tools/cephfs: always execute scan_{extents,inodes,frags} and cleanup

Even when the number of objects reported from pool stats is zero.
Pool stats metrics are delayed and cannot be fully relied on for
accuracy. Trusting the number of objects (esp. when reported as
zero) could result in missed steps during data-scan execution.

So, we try to do two things now:

1. ProgressTracker::display_progress() will display progress only
when the total items exceeds to the number of progress items.
2. Refresh total object count during each iteration of processing
objects. This might be a bit too much, so we probably need to
do this periodically rather than on each iteration.

Fixes: http://tracker.ceph.com/issues/75083
Signed-off-by: Venky Shankar <vshankar@redhat.com>