lib/test_hmm.c: handle src_pfns and dst_pfns allocation failure

In the Linux kernel, the following vulnerability has been resolved:

lib/test_hmm.c: handle src_pfns and dst_pfns allocation failure

The kcalloc() in dmirror_device_evict_chunk() will return null if the

physical memory has run out. As a result, if src_pfns or dst_pfns is

dereferenced, the null pointer dereference bug will happen.

Moreover, the device is going away. If the kcalloc() fails, the pages

mapping a chunk could not be evicted. So add a __GFP_NOFAIL flag in

kcalloc().

Finally, as there is no need to have physically contiguous memory, Switch

kcalloc() to kvcalloc() in order to avoid failing allocations.