DDF: Write new conf entries with a single write.

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>