xfs_scrub: redistribute read verify pool flush and destroy responsibilities

xfs_scrub: redistribute read verify pool flush and destroy responsibilities

Since workqueues now have separate primitives for "wait for all queued
work" and "destroy workqueue", it makes more sense for the read verify
pool code to call the workqueue destructor from its own destructor
function.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>