prog: adjust number of image mutations
prog: adjust number of image mutations

Also adjust number of mutations per image.
We do one per 128K of interesting data. But we have no single seed
that has so much interesting data. Here is the amount of interesting data in seeds:
https://gist.github.com/dvyukov/566b20364610d80e5d0534524546c3f0
Reduce this cuhnk size to 4K.

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