xfs_scrub: balance inode chunk scan across CPUs

xfs_scrub: balance inode chunk scan across CPUs

Use the bounded workqueue functionality to spread the inode chunk scan
load across the CPUs more evenly.  First, we create per-AG workers to
walk each AG's inode btree to create inode batch work items for each
inobt record.  These items are added to a (second) bounded workqueue
that invokes BULKSTAT and invokes the caller's function on each bulkstat
record.

By splitting the work items into batches of 64 inodes instead of one
thread per AG, we keep the level of parallelism at a reasonably high
level almost all the way to the end of the inode scan if the inodes are
not evenly divided across AGs or if a few files have far more extent
records than average.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>