Since we know who's holding the lock, and we're gonna have to yield anyway,
we can just ask the scheduler to donate any remaining ticks to that process.
Also add assertion in Lock that the scheduler isn't currently active.
I've been seeing occasional fuckups that I suspect might be someone called
by the scheduler trying to take a busy lock.
The SpinLock was all backwards and didn't actually work. Fixing it exposed
how wrong most of the locking here is.
I need to come up with a better granularity here.
Add a separate lock to protect the VFS. I think this might be a good idea.
I'm not sure it's a good approach though. I'll fiddle with it as I go along.
It's really fun to figure out all these things on my own.
