io_uring: allow allocated fixed files for openat/openat2

io_uring: allow allocated fixed files for openat/openat2

If the applications passes in UINT_MAX as the file_slot, then that's a
hint to allocate a fixed file descriptor rather than have one be passed
in directly.

This can be useful for having io_uring manage the direct descriptor space.

Normal open direct requests will complete with 0 for success, and < 0
in case of error. If io_uring is asked to allocated the direct descriptor,
then the direct descriptor is returned in case of success.

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