io_uring: refactor tctx_task_work

Merge almost identical sections of tctx_task_work(), this will make code
modifications later easier and also inlines handle_tw_list().

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/d06592d91e3e7559e7a4dbb8907d110863008dc7.1674484266.git.asml.silence@gmail.com


[axboe: fold in setting count to zero patch from Tom Rix]
Signed-off-by: Jens Axboe <axboe@kernel.dk>