[proof-of-concept/rfc] per object file mmaps

The 2+3 patches are actually the code, the first was just a cleanup.

Anyways the second patch describes it best, but I've taken the approach that we just need to keep track of what filp this object has had a handle created on, and block mmaps on it. We could also probably block a bit more explicitly in the driver respective mmap offset retrieval ioctls, however we need to block in mmap itself to stop people just picking misc address and trying them.

It doesn't really add much now, but with render nodes where we have flink_to or fd passing it would be more useful.

I've no idea if I'll get to do much more with this, so consider it an indication of how I'd like it done for someone else to run with :-)

Dave.