virtio: fix memory leak in virtqueue_add()

virtio: fix memory leak in virtqueue_add()

Orabug: 26813294

When using the indirect buffers feature, 'desc' is allocated in
virtqueue_add() but isn't freed before leaving on a ring full error,
causing a memory leak.

For example, it seems rather clear that this can trigger
with virtio net if mergeable buffers are not used.

Cc: stable@vger.kernel.org
Signed-off-by: Wei Yongjun <weiyj.lk@gmail.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
(cherry picked from commit 58625edf9e2515ed41dac2a24fa8004030a87b87)
Signed-off-by: Si-Wei Liu <si-wei.liu@oracle.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>