LU-14514 flr: mirror split should not make stale file

LU-14514 flr: mirror split should not make stale file

Mirror split could leave an all stale mirrors file, this patch
prevent removing the last non-stale mirror from the file.

Lustre-change: https://review.whamcloud.com/42024
Lustre-commit: 83c790cbf2f8f7452e1382051564af6f155b47cf

Signed-off-by: Bobi Jam <bobijam@whamcloud.com>
Change-Id: I63007784929a2cd18d2823e2250f7307ca7d8d45
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: John L. Hammond <jhammond@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/46089
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Andreas Dilger <adilger@whamcloud.com>