mm/hmm: add a test for cross device private faults

Add a simple test case for when hmm_range_fault() is called with the
HMM_PFN_REQ_FAULT flag and a device private PTE is found for a device
other than the hmm_range::dev_private_owner.  This should cause the page
to be faulted back to system memory from the other device and the PFN
returned in the output array.

Also, remove a piece of code that unnecessarily unmaps part of the buffer.

Link: https://lkml.kernel.org/r/20220727000837.4128709-3-rcampbell@nvidia.com
Link: https://lkml.kernel.org/r/20220725183615.4118795-3-rcampbell@nvidia.com
Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
Reviewed-by: Alistair Popple <apopple@nvidia.com>
Cc: Felix Kuehling <felix.kuehling@amd.com>
Cc: Philip Yang <Philip.Yang@amd.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>