QNetworkReplyImpl: optimize QRingBuffer allocation

Use QSharedPointer<T>::create(), which co-locates the refcount with the payload
in a single memory allocation, instead of QSharedPointer<T>(new T), which causes
two allocations.

Change-Id: I84980e98c4fe9773f12533983937eafa0b0ab250
Reviewed-by: Peter Hartmann <phartmann@blackberry.com>