Bring back mmap support. Takes 'fsx' from hanging, to failing. Output of
```
data miscompare @ 119488
OFFSET     GOOD       BAD        LENGTH     BADOP#   Last: WRITE    TRUNC-   TRUNC+
0x0001d2c0 0x00000004 0x00000000 0x0000011c -1             4        -1       2
```