[$] Cleanup on aisle fsconfig()

As part of the process of writing man pages for the “new” mount API, which has been available in the
kernel since 2019, Aleksa Sarai encountered a number of places where the fsconfig()
system call—for configuring filesystems before mounting—needs to be cleaned up. In the 2025 Linux Plumbers Conference
(LPC) session
that he led, Sarai wanted to discuss some of the problems he found,
including at least one with security implications. The idea of the session
was for him to describe the various bugs and ambiguities that he had found,
but he also wanted attendees to raise other problems they had with the
system call.