[$] An io_uring-based user-space block driver

The addition of the ublk driver during the 6.0 merge window would have been
easy to miss; it was buried deeply within an io_uring pull request and is
entirely devoid of any sort of documentation that might indicate why it
merits a closer look. Ublk is intended to facilitate the implementation of
high-performance block drivers in user space; to that end, it uses io_uring
for its communication with the kernel. This driver is considered
experimental for now; if it is successful, it might just be a harbinger of
more significant changes to come to the kernel in the future.