io_uring/msg_ring: improve handling of target CQE posting

Use the exported helper for queueing task_work for message passing,
rather than rolling our own.

Note that this is only done for strict data messages for now, file
descriptor passing messages still rely on the kernel task_work. It could
get converted at some point if it's performance critical.

This improves peak performance of message passing by about 5x in some
basic testing, with 2 threads just sending messages to each other.
Before this change, it was capped at around 700K/sec, with the change
it's at over 4M/sec.

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