Optional mandatory locking

We disable all sorts of stuff in the kernel these days in the name of "tinification" so I didn't see that allowing people to compile this out was really any different. FWIW, while I was doing some overhaul of the file locking code a while back, I did see how we could make mandatory locking race free (or at least, less racy), but I found it really hard to care enough to do the work for it.

It has always seemed like a bit of a hack anyway (you need a mount option _and_ a special mode-bit combo which is not at all intuitive), and the use-cases for it are pretty thin on the ground.