xfs_scrub: scan whole-fs metadata files in parallel

xfs_scrub: scan whole-fs metadata files in parallel

The realtime bitmap and the three quota files are completely independent
of each other, which means that we ought to be able to scan them in
parallel.  Rework the phase2 code so that we can do this.  Note,
however, that the realtime summary file summarizes the contents of the
realtime bitmap, so we must coordinate the workqueue threads.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bill O'Donnell <bodonnel@redhat.com>