io_uring: move submitter_task out of cold cacheline

->submitter_task is used somewhat more frequent now than before, i.e.
for local tw enqueue and run, let's move it from the end of ctx, which
is full of cold data, to the first cacheline with mostly constants.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/415ca91dc5ad1dec612b892e489cda98e1069542.1673274244.git.asml.silence@gmail.com


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