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

[Dave Airlie]

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.