zero the entire journal instead of just the first block

zero the entire journal instead of just the first block

I'm seeing some weeeeird stuff with sparse ext4 images where
files sometimes seem to come back from the dead after reflashing,
etc.  My crazy theory is they are hiding in the journal.

Colin humored me and pointed out how to ensure it's entirely
zero'd.

Change-Id: I4d14a2d4879c10540ef3c677ceb523bc061c1ec1