buf_ring in HEAD is racy

[Adrian Chadd]

Also, I think it's a good idea to get a bullet-proof bufring
implementation that uses atomics rather than mutexes. We really want
there to be a cheap way to insert things into the queue.

Honestly, we could just do with a generic multi-writer, single-reader
ring type that we can re-use for other things, rather than just mbufs.
We could also have the code run in userland and do some very in-depth
stress testing of it there rather than needing it to be in the kernel.

Hm..


-adrian