selftests/udmabuf: add tests to verify data after page migration

Since the memfd pages associated with a udmabuf may be migrated as part of
udmabuf create, we need to verify the data coherency after successful
migration.  The new tests added in this patch try to do just that using 4k
sized pages and also 2 MB sized huge pages for the memfd.

Successful completion of the tests would mean that there is no disconnect
between the memfd pages and the ones associated with a udmabuf.  And,
these tests can also be augmented in the future to test newer udmabuf
features (such as handling memfd hole punch).

The idea for these tests comes from a patch by Mike Kravetz here:
https://lists.freedesktop.org/archives/dri-devel/2023-June/410623.html

v1->v2: (suggestions from Shuah)
- Use ksft_* functions to print and capture results of tests
- Use appropriate KSFT_* status codes for exit()
- Add Mike Kravetz's suggested-by tag

Link: https://lkml.kernel.org/r/20240624063952.1572359-10-vivek.kasireddy@intel.com
Signed-off-by: Vivek Kasireddy <vivek.kasireddy@intel.com>
Suggested-by: Mike Kravetz <mike.kravetz@oracle.com>
Acked-by: Dave Airlie <airlied@redhat.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Hugh Dickins <hughd@google.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Dongwon Kim <dongwon.kim@intel.com>
Cc: Junxiao Chang <junxiao.chang@intel.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Oscar Salvador <osalvador@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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