set default journal mode to TRUNCATE, not PERSIST

PERSIST causes data to linger in the journal beyond transactions, so
it interacts poorly with SECURE_DELETE. It's also not necessarily
faster, especially since it makes the page cache and SSD cache (if there
is one) colder by holding on to blocks that are no longer used.

The default chosen by upstream is DELETE, but TRUNCATE is a bit lighter
since it doesn't need to create/delete the file.

Change-Id: If790271c253a840df241980f538fc0d844ba7857