Revert "vfio/type1: Unpin zero pages"

This reverts commit 873aefb3.

This was a heinous workaround and it turns out it's been fixed in mm
twice since it was introduced.  Most recently, commit c8070b78
("mm: Don't pin ZERO_PAGE in pin_user_pages()") would have prevented
running up the zeropage refcount, but even before that commit
84209e87 ("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 <david@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Link: https://lore.kernel.org/r/20240229223544.257207-1-alex.williamson@redhat.com


Signed-off-by: Alex Williamson <alex.williamson@redhat.com>