tools/syz-imagegen: update `vfat` image generation
tools/syz-imagegen: update `vfat` image generation

`./syz-imagegen -fs vfat` now fails for some images with the following message:
  mkfs.vfat: Not enough or too many clusters for filesystem - try less or more sectors per cluster
This error appears only for the runs with sectors per cluster set to `-s 128`.

This commit takes the error message's advice by reducing sectors per
cluster to `-s 64`. Images will be regenerated in a future commit.

xairy/syzkaller,google/syzkaller,xairy/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,google/syzkaller