io_uring/msg_ring: refactor a few helper functions

Mostly just to skip them taking an io_kiocb, rather just pass in the
ctx and io_msg directly.

In preparation for being able to issue a MSG_RING request without
having an io_kiocb. No functional changes in this patch.

Link: https://lore.kernel.org/r/20240924115932.116167-2-axboe@kernel.dk


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