An interrupted dma_fence_wait() becomes an -ERESTARTSYS returned
to userspace ioctl(DRM_IOCTL_VIRTGPU_EXECBUFFER) calls, prompting to
retry the ioctl(), but the passed exbuf->fence_fd has been reset to -1,
making the retry attempt fail at sync_file_get_fence().

The uapi for DRM_IOCTL_VIRTGPU_EXECBUFFER is changed to retain the
passed value for exbuf->fence_fd when returning anything besides a
successful result from the ioctl.

Fixes: 2cd7b6f0 ("drm/virtio: add in/out fence support for explicit synchronization")
Signed-off-by: NRyan Neph <ryanneph@chromium.org>
Reviewed-by: NRob Clark <robdclark@gmail.com>
Reviewed-by: NDmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: NDmitry Osipenko <dmitry.osipenko@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230203233345.2477767-1-ryanneph@chromium.org