[Nouveau] turn the hmm migrate_vma upside down

[Christoph Hellwig]

On Tue, Jul 30, 2019 at 12:32:24PM +0000, Jason Gunthorpe wrote:
> Does this only impact hmm users, or does migrate.c have a broader
> usage?

migrate.c really contains two things:  the traditional page migration
code implementing aops ->migrate semantics, and migrate_vma and its
callbacks.  The first part is broader, the second part is hmm specific
(and should probably have been in a file of its own, given that it is
guarded off CONFIG_MIGRATE_VMA_HELPER).  This series only touched the
latter part.