nouveau/dmem: handle kcalloc() allocation failure

In the Linux kernel, the following vulnerability has been resolved: nouveau/dmem: handle kcalloc() allocation failure The kcalloc() in nouveau_dmem_evict_chunk() will return null if the physical memory has run out. As a result, if we dereference src_pfns, dst_pfns or dma_addrs, the null pointer dereference bugs will happen. Moreover, the GPU is going away. If the kcalloc() fails, we could not evict all pages mapping a chunk. So this patch adds a __GFP_NOFAIL flag in kcalloc(). Finally, as there is no need to have physically contiguous memory, this patch switches kcalloc() to kvcalloc() in order to avoid failing allocations.