xfs_scrub: bump work_threads to include the controller thread

xfs_scrub: bump work_threads to include the controller thread

Bump @work_threads in the scrub phase setup function because we will
soon want the main thread (i.e. the one that coordinates workers) to be
factored into per-thread data structures.  We'll need this in an
upcoming patch to render error string prefixes to preallocated
per-thread buffers.

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>