[$] A new filesystem for pidfds

The pidfd abstraction is a Linux-specificway of referring to processes that avoids the race conditions inherent inUnix process ID numbers. Since a pidfd is a file descriptor, it needs afilesystem to implement the usual operations performed on files. As theuse of pidfds has grown, they have stressed the limits of the simplefilesystem that was created for them. Christian Brauner has createda new filesystem for pidfds that seems likely to debut in the 6.9kernel, but it ran into a little bump along the way, demonstrating thatthings you cannot see can still hurt you.