io_uring: register single issuer task at creation

Instead of picking the task from the first submitter task, rather use the
creator task or in the case of disabled (IORING_SETUP_R_DISABLED) the
enabling task.

This approach allows a lot of simplification of the logic here. This
removes init logic from the submission path, which can always be a bit
confusing, but also removes the need for locking to write (or read) the
submitter_task.

Users that want to move a ring before submitting can create the ring
disabled and then enable it on the submitting task.

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>