vmbus: add prefetch to ring buffer iterator

When iterating over incoming ring elements from the host, prefetch
the next descriptor so that it is cache hot.

Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>