xfs_scrub: fix nr_dirs accounting problems

xfs_scrub: fix nr_dirs accounting problems

When we're scanning the directory tree, we bump nr_dirs every time we
think we're going to queue a new directory to process, and we decrement
it every time we're finished doing something with a directory
(successful or not).  We forgot to undo a counter increment when
workqueue_add fails, so refactor the code into helpers and call them
as necessary for correct operation.

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