[$] API changes for the futex robust list

The robustfutex kernel API is a way for a user-space program to ensure that thelocks it holds are properly cleaned up when it exits. But the API suffersfrom a number of different problems, as Andre Almeida described in a session in the"Gaming onLinux" microconference at the 2025 Linux Plumbers Conference in Tokyo.He had some ideas for a new API that would solve many of those problems,which he wanted to discuss with attendees; there is adifficult-to-trigger race condition that he wanted to talk about too.