io_uring: simplify io_req_link_next()

"if (nxt)" is always true, as it was checked in the while's condition.
io_wq_current_is_worker() is unnecessary, as non-async callers don't
pass nxt, so io_queue_async_work() will be called for them anyway.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>