io_uring/msg_ring: flag target ring as having task_work, if needed

Before the recent change, we didn't even wake the targeted task when
posting the cqe remotely. Now we do wake it, but we do want to ensure
that the recipient knows there's potential work there that needs to
get processed to get the CQE posted.

OR in IORING_SQ_TASKRUN for that purpose.

Link: https://lore.kernel.org/io-uring/2843c6b4-ba9a-b67d-e0f4-957f42098489@kernel.dk/
Fixes: 6d043ee1

 ("io_uring: do msg_ring in target task via tw")
Signed-off-by: Jens Axboe <axboe@kernel.dk>