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.