io_uring/net: switch io_send() and io_send_zc() to using io_async_msghdr

No functional changes in this patch, just in preparation for carrying
more state then what is being done now, if necessary. While unifying
some of this code, add a generic send setup prep handler that they can
both use.

This gets rid of some manual msghdr and sockaddr on the stack, and makes
it look a bit more like the sendmsg/recvmsg variants. Going forward, more
can get unified on top.

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