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>
3 files changed