drm/syncobj: Fix handle <-> fd ioctls with dirty stack

Consider the following application:

    #include <fcntl.h>
    #include <string.h>
    #include <drm/drm.h>
    #include <sys/ioctl.h>

    int main(void) {
        int fd = open("/dev/dri/renderD128", O_RDWR);
        struct drm_syncobj_create arg1;
        ioctl(fd, DRM_IOCTL_SYNCOBJ_CREATE, &arg1);
        struct drm_syncobj_handle arg2;
        memset(&arg2, 1, sizeof(arg2)); // simulate dirty stack
        arg2.handle = arg1.handle;
        arg2.flags = 0;
        arg2.fd = 0;
        arg2.pad = 0;
        // arg2.point = 0; // userspace is required to set point to 0
        ioctl(fd, DRM_IOCTL_SYNCOBJ_HANDLE_TO_FD, &arg2);
    }

The last ioctl returns EINVAL because args->point is not 0. However,
userspace developed against older kernel versions is not aware of the
new point field and might therefore not initialize it.

The correct check would be

    if (args->flags & DRM_SYNCOBJ_FD_TO_HANDLE_FLAGS_TIMELINE)
        return -EINVAL;

However, there might already be userspace that relies on this not
returning an error as long as point == 0. Therefore use the more lenient
check.

Fixes: c2d3a730 ("drm/syncobj: Extend EXPORT_SYNC_FILE for timeline syncobjs")
Signed-off-by: Julian Orth <ju.orth@gmail.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Link: https://lore.kernel.org/r/20260301-point-v1-1-21fc5fd98614@gmail.com