LU-16979 utils: enable throttling mirror extend

LU-16979 utils: enable throttling mirror extend

this can be useful in some scenarios like massive mirror
creation.

Test-Parameters: trivial
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: Ia84054f3519cd5cef37aaabb2ae605fb6ea200e0
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/51758
Reviewed-by: Timothy Day <timday@amazon.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>