hwrng: virtio - Fix race on data_avail and actual data

In the Linux kernel, the following vulnerability has been resolved:

hwrng: virtio - Fix race on data_avail and actual data

The virtio rng device kicks off a new entropy request whenever the

data available reaches zero. When a new request occurs at the end

of a read operation, that is, when the result of that request is

only needed by the next reader, then there is a race between the

writing of the new data and the next reader.

This is because there is no synchronisation whatsoever between the

writer and the reader.

Fix this by writing data_avail with smp_store_release and reading

it with smp_load_acquire when we first enter read. The subsequent

reads are safe because they're either protected by the first load

acquire, or by the completion mechanism.

Also remove the redundant zeroing of data_idx in random_recv_done

(data_idx must already be zero at this point) and data_avail in

request_entropy (ditto).