LU-16842 fsx: tolerate delete last non-stale mirror error

LU-16842 fsx: tolerate delete last non-stale mirror error

fsx mirror split test could try to delete the last non-stale mirror
of a file and that's a tolerable error scenario. The fsx FLR test
randomly choose a mirror operation and this situation could happen.

Lustre-change: https://review.whamcloud.com/51090
Lustre-commit: TBD (from 9bc22dcefbe3a7ac66a3e8f2ad042f1e018d45a9)

Fixes: 04ab0cc869c (LU-14156 utils: mirror split to check for last in-sync early)
Signed-off-by: Bobi Jam <bobijam@whamcloud.com>
Change-Id: I80c294da80740b21e00ae72a092fd8883ec7d60e
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/51092
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>