The recent change to skip over invalid conf entries was bad because
it could leave garbage on the disk.
But we don't to write each entry separately as the writes a O_DIRECT
and so synchronous so it takes way too long.
So allocate a large buffer (probably the one used to read the config records)
and fill that then write it all at once.
Reported-by: Martin Wilck <mwilck@arcor.de> Signed-off-by: NeilBrown <neilb@suse.de>