dma-buf: improve sg_table debugging hack v4

This debugging hack is important to enforce the rule that importers
should *never* touch the underlying struct page of the exporter.

Instead of just mangling the page link create a copy of the sg_table
but only copy over the DMA addresses and not the pages.

This will cause a NULL pointer de-reference if the importer tries to
touch the struct page. Still quite a hack but this at least allows the
exporter to properly keeps it's sg_table intact while allowing the
DMA-buf maintainer to find and fix misbehaving importers and finally
switch over to using a different data structure in the future.

v2: improve the hack further by using a wrapper structure and explaining
the background a bit more in the commit message.
v3: fix some whitespace issues, use sg_assign_page().
v4: give the functions a better name

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Link: https://lore.kernel.org/r/20251205130604.1582-1-christian.koenig@amd.com

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