io_uring/net: don't overconsume buffers when using MSG_TRUNC

In the Linux kernel, the following vulnerability has been resolved: io_uring/net: don't overconsume buffers when using MSG_TRUNC When a recv/recvmsg is issued with MSG_TRUNC and the incoming packet is larger than the provided buffer, the net layer returns the full length of the packet rather than the number of bytes actually copied into the buffer. As a result, io_uring advances more of the provided buffer ring than was actually filled. Use the actual filled region size to consume the buffer, but still return the full size to preserve MSG_TRUNC semantics. Take care with multishot, because that seems to already truncate the consumption based on the available payload size. This was reported in https://github.com/axboe/liburing/issues/1619. [axboe: fold in size_t unsigned fix]