From: Andries.Brouwer@cwi.nl

ext2 used a 32-bit field for dev_t, with possibly undefined storage
following; thus, no action was required to go to 32-bit dev_t, but going to
64-bit dev_t required some subtlety: 0 was written in the first word and
the 64 bits in the following two.  Al truncated my 64-bit stuff to 32 bits
but did not understand why there was this split, and wrote 0 followed by a
single word.  We should at least zero the word following to have
well-defined storage later.