LU-17814 utils: add parallel find arg

LU-17814 utils: add parallel find arg

lfs find is currently serial, but it can be much faster if
we add a parallel implementation where it creates a work
unit for every directory and handles them in parallel.

This is the first patch of that implementation.

Add arg to control thread count.
No implementation yet.

Test-Parameters: trivial
Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I73b542decd75e1f7580f584d2ea6a219af51be4c
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/57221
Reviewed-by: Marc Vef <mvef@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>