We now treat the block device as fixed-size rather than trying to grow it in response
to kernel activity.
Previously, if you tried to create 2 files in a row, only the first 1 would actually appear,
because the device only ever got larger when a byte was written immediately past the end of
the device.
Now we just let the kernel write bytes to the disk anywhere, so new files can be created even
when the previous file is not completely full.
Also, fix buffer overrun in reading filesystem image, and use a more idiomatic fgetc loop.
1b8b113536