io_uring: refactor io_submit_flush_completions()

struct io_comp_state is always contained in struct io_ring_ctx, don't
pass them into io_submit_flush_completions() separately, it makes the
interface cleaner and simplifies it for the compiler.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/44d6ca57003a82484338e95197024dbd65a1b376.1623949695.git.asml.silence@gmail.com


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