io_uring: simplify __io_uring_add_tctx_node

Remove submitter parameter from __io_uring_add_tctx_node.

It was only called from one place, and we can do that logic in that one
place.

Signed-off-by: Dylan Yudaken <dylany@fb.com>
Fixes: 97bbdc06 ("io_uring: add IORING_SETUP_SINGLE_ISSUER")
Signed-off-by: Jens Axboe <axboe@kernel.dk>