xfs_scrub: abort all read verification work immediately on error

xfs_scrub: abort all read verification work immediately on error

Add a new abort function to the read verify pool code so that the caller
can immediately abort all pending verification work if things start
going wrong.  There's no point in waiting for queued work to run if
we've already decided to bail.

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>