io_uring/net: initalize msghdr->msg_inq to known value

We can't currently tell if ->msg_inq was set when we ask for msg_get_inq,
initialize it to -1U so we can tell apart if it was set and there's
no data left, or if it just wasn't set at all by the protocol.

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