xen/blkfront: fix bug in backported patch

The backport of commit 8f5a695d99000fc ("xen/blkfront: don't take local
copy of a request from the ring page") to stable 4.4 kernel introduced
a bug when adding the needed blkif_ring_get_request() function, as
info->ring.req_prod_pvt was incremented twice now.

Fix that be deleting the now superfluous increments after calling that
function.

Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>