shared shadow vm object invalidation regression test

Add a regression test for a scenario where a shadow vm object is shared
by multiple mappings. If a page COW occurs through one of the mappings,
then the virtual-to-physical mapping may become invalidated.

This tests the scenario from CVE-2021-29626 which was fixed by
982693bb729badac4e65ecd59772979f2849a2b2.

Reviewed by: markj
Sponsored by: Dell EMC Isilon

(cherry picked from commit 13d4f96130b64b42f74dfc484305b08c4da54669)