mm: gup: introduce PageAnonGup helpers This uses the same technique as David's PageAnonExclusive. The implementation in turn is based on upstream commit 78fbe906cc900b33ce078102e13e0e99b5b8c406 . (cherry picked from commit 78fbe906cc900b33ce078102e13e0e99b5b8c406) Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>