newttm: ttm_buffer_object_transfer last take

[Jerome G. <gl...@fr...>]

Hi Thomas,

There is another bug in ttm_buffer_object_transfer
when we copy *fbo = *bo we should set the fbo->vm_node
to NULL otherwise the vm_node will be free with the
ghost object and bad things will happen for the still
alive object.

Cheers, 
Jerome Glisse <gl...@fr...>