Fix a bug when start linux0.11 with "qemu-system-i386 -L pc-bios -m 16 -boot a -fda bootimage -fdb rootfsimage".

N/A

The fist floppy used to boot kernel image, then load super block from
the second floppy need set super block number is "1".

Signed-off-by: liminghao <liminghao@xiaomi.com>