mmap file read fails in gem5 syscall emulation

As of LKMC 6936bd6ba996dee40f7cd826e5cf01ef39c2cabf and gem5 master 8549ee4a6dfc86a941cee0a478c01f6f2c146c3c:

```
./run -ax -u userland/posix/mmap_file.c --emulator gem5 --gem5-worktree master --static
```

the assertion for the correct read value fails with:

```
mmap_file.out: /path/to/linux-kernel-module-cheat/userland/posix/mmap_file.c:92: main: Assertion `map[1] == 1' failed.
```
