Revert "vfio/type1: Unpin zero pages"

This reverts commit 873aefb376bbc0ed1dd2381ea1d6ec88106fdbd4.

This was a heinous workaround and it turns out it's been fixed in mm
twice since it was introduced.  Most recently, commit c8070b787519
("mm: Don't pin ZERO_PAGE in pin_user_pages()") would have prevented
running up the zeropage refcount, but even before that commit
84209e87c696 ("mm/gup: reliable R/O long-term pinning in COW mappings")
avoids the vfio use case from pinning the zeropage at all, instead
replacing it with exclusive anonymous pages.

Remove this now useless overhead.

Suggested-by: David Hildenbrand <[email protected]>
Reviewed-by: David Hildenbrand <[email protected]>
Reviewed-by: Kevin Tian <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alex Williamson <[email protected]>

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