LU-18560 utils: Fix ldiskfs_write_dd Coverity warnings

LU-18560 utils: Fix ldiskfs_write_dd Coverity warnings

Coverity has reported issues with the "ldiskfs_write_ldd()" function.
In particular OVERRUN, INTEGER_OVERFLOW, NO_EFFECT, and
ARRAY_VS_SINGLETON warnings.

This patch fixes these warnings.

Fixes: 7687e22af35d ("LU-9936 utils: Improve ldiskfs_write_dd() writing mount data")
Signed-off-by: Marc Vef <mvef@whamcloud.com>
Change-Id: Id1379d9c6d53e697ff2ee12570b4569c4bce5e73
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/57449
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Mikhail Pershin <mpershin@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>