io_uring: use more specific type in rcv/snd msg cp

send/recv msghdr initialisation works with struct io_async_msghdr, but
pulls the whole struct io_async_ctx for no reason. That complicates it
with composite accessing, e.g. io->msg.

Use and pass the most specific type, which is struct io_async_msghdr.
It is the larget field in union io_async_ctx and doesn't save stack
space, but looks clearer.
The most of the changes are replacing "io->msg." with "iomsg->"

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>