watch_mount(), watch_sb(), and fsinfo() (again)

This doesn’t really seem like a big deal - novel and different mechanisms are a problem when they’re, well, novel and different. This is just a pipe, and unless I’ve missed something, it’s not requiring implementing anything.

I mean maybe netlink would be superior, but this seems nicely trivial. It’s not like they’re knocking up a netlink competitor, just using a trivial interface where they can.