io_uring/msg_ring: tighten requirement for remote posting

Currently this is gated on whether or not the target ring needs a local
completion - and if so, whether or not we're running on the right task.
The use case for same thread cross posting is probably a lot less
relevant than remote posting. And since we're going to improve this
situation anyway, just gate it on local posting and ignore what task
we're currently running on.

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