io_uring: restrict IORING_SETUP_SQPOLL to root

This options spawns a kernel side thread that will poll for submissions
(and completions, if IORING_SETUP_IOPOLL is set). As this allows a user
to potentially use more cycles outside of the normal hierarchy,
restrict the use of this feature to root.

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