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.
BlockDevice was the wrong name for this abstraction, since a block device
is a type of file in a unix system, and we should use that name for that
concept in the fs implementation.
