vhost mergeable buffers guest's cgroup

Sridhar, David,
I pushed a mergeable buffers patchset on my vhost-net-next branch, on top of the threading changes.

This is a minimal patch, which in my testing has zero impact on non-mergeable path.

Please give this a spin and let me know.

Thanks!

MST

The userspace bits can be found on my qemu-kvm tree at kernel.org, branch vhost_mergeable.
Please use these for testing.