engines/io_uring: add support for registered files

engines/io_uring: add support for registered files

This feature is exposed as a separate option, like fixedbufs, and
provides a way for fio to register a set of files with the kernel.
This improves IO efficiency.

It is also a requirement to be able to use sqthread_poll, as that
feature requires fixed files on the kernel side.

Signed-off-by: Jens Axboe <axboe@kernel.dk>