LU-14156 utils: mirror split to check for last in-sync early

LU-14156 utils: mirror split to check for last in-sync early

currently this check to prevent last in-sync component is done
once the file is open with O_RDWR which interrupts on-going
resync/extend process. instead we can do this check early once
the layout is fetched (after the first open with O_RDONLY).

Lustre-change: https://review.whamcloud.com/40782
Lustre-commit: TBD (from 9996c22e346c219df5b165eb5ceab09873303bc2)

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: Iee08d23008b44d2a7b2127358116a95ace40b7dd
Reviewed-on: https://review.whamcloud.com/47058
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>