selftests/hmm-tests: add test for dirty bits

selftests/hmm-tests: add test for dirty bits

We were not correctly copying PTE dirty bits to pages during
migrate_vma_setup() calls.  This could potentially lead to data loss, so
add a test for this.

Link: https://lkml.kernel.org/r/23069a5c6e07d16d4c4f0951ff003591ffc4f656.1660635033.git-series.apopple@nvidia.com
Signed-off-by: Alistair Popple <apopple@nvidia.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Huang Ying <ying.huang@intel.com>
Cc: Alex Sierra <alex.sierra@amd.com>
Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Felix Kuehling <felix.kuehling@amd.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Karol Herbst <kherbst@redhat.com>
Cc: Logan Gunthorpe <logang@deltatee.com>
Cc: Lyude Paul <lyude@redhat.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Paul Mackerras <paulus@ozlabs.org>
Cc: Ralph Campbell <rcampbell@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>