xfs_scrub: fix threadcount estimates for phase 6

xfs_scrub: fix threadcount estimates for phase 6

If a filesystem has a realtime device or an external log device, the
media scan can start up a separate readverify controller (and workqueue)
to handle that.  Each of those controllers can call progress_add, so we
need to bump up nr_threads so that the progress reports controller knows
to make its ptvar big enough to handle all these threads.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>