vdpa: Fix error logic in vdpa_nl_cmd_dev_get_doit

In vdpa_nl_cmd_dev_get_doit(), if the call to genlmsg_reply() fails we
must not call nlmsg_free() since this is done inside genlmsg_reply().

Fix it.

Fixes: bc0d90ee ("vdpa: Enable user to query vdpa device info")
Reviewed-by: Si-Wei Liu <si-wei.liu@oracle.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Eli Cohen <elic@nvidia.com>
Message-Id: <20220518133804.1075129-2-elic@nvidia.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>