io_uring: optimise tctx node checks/alloc

First of all, w need to set tctx->sqpoll only when we add a new entry
into ->xa, so move it from the hot path. Also extract a hot path for
io_uring_add_task_file() as an inline helper.

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