LU-10070 utils: setstripe component-add support for SEL

LU-10070 utils: setstripe component-add support for SEL

the math of the SEL component sizes calculation does not work for the
component end when some components already exist.

Signed-off-by: Vitaly Fertman <c17818@cray.com>
Change-Id: I793ea7e5a5ac8f4639f694f83af7d413bd2b982c
Reviewed-on: https://review.whamcloud.com/35314
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Alexey Lyashkov <c17817@cray.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>