LU-9936 utils: Improve ldiskfs_write_dd() writing mount data

LU-9936 utils: Improve ldiskfs_write_dd() writing mount data

Writing the mount data to a file used buffered stream I/O missing a
flush operation with incomplete error handling.

This patch replaces buffered stream I/O with the Unix I/O API and adds
error handling.

Signed-off-by: Marc Vef <mvef@whamcloud.com>
Change-Id: Ia7dfc186b1c6f581dee461bd277f07dcc2eabafa
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/56408
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Mikhail Pershin <mpershin@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>