io_uring: use syscall helpers for the hot path

io_uring: use syscall helpers for the hot path

The only real hot system call here is the io_uring_enter(2) call,
as that'll happen during the IO submission/completion parts. The rest
are just setup function calls, we don't really care about those.

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