The problematic kthread freezer

Well, I handwaved that a bit. It _is_ rather tricky to wire up for NFS.

Basically what I think we'd want to do is to have fsfreeze tell the RPC transport layer that it should stop sending RPCs to the server(s) and drain the queue by waiting on replies to come in.

The question though is what to do with threads sitting in syscalls that need to issue an RPC. "Parking" them down at the layer where we're synchronously waiting for an RPC reply would be bad, as it would mean that we could easily be holding vfs-layer locks at that point (inode->i_rwsem for instance).

How should that work?