simg_dump.py: fix error for CHUNK_TYPE_FILL

In python3, division of two integers produces a float,
which cannot be used to multiply a bytes sequence.

Using // to produce an integer quotient.

Bug: 245257511
Test: ./system/core/libsparse/simg_dump.py -sv path/to/image
Change-Id: Ibe95056409ac3676a68f5e7693331ca4a66e9fb7