disk wiping: fix bug in chunk size computation

The current wipe_chunk_size computation is doing min(int_value,
float_value). For small disks (below 10GiB), the actual formula will
result into the float value being chosen. This results into very
interesting behaviour:

Wiping disk 0, offset 102.4, chunk 102.4
Wiping disk 0, offset 204.8, chunk 102.4
…
Wiping disk 0, offset 921.6, chunk 102.4
Wiping disk 0, offset 1024.0, chunk 1.13686837722e-13

Since these are passed to dd via %d, this will result into the call to
dd specifying offset 1024 and count 0, which will fail.

We just need to enforce conversion to int, in order to not get bitten
by floating point rounding errors.

The patch also reorders some logging messages in order to log the
chunk size.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>