LU-9897 utils: allow setting llverfs subdir count

LU-9897 utils: allow setting llverfs subdir count

Allow specifying the subdirectory count directly rather
than calculating it based on the filesystem size.

Test-Parameters: trivial testlist=conf-sanity
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Idcae188ef4bdb417f0f983718bce7e55093ebbe5
Reviewed-on: https://review.whamcloud.com/39347
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Anjus George <georgea@ornl.gov>
Reviewed-by: Oleg Drokin <green@whamcloud.com>