[Nouveau] turn hmm migrate_vma upside down v2

[Christoph Hellwig]

Hi Jérôme, Ben and Jason,

below is a series against the hmm tree which starts revamping the
migrate_vma functionality.  The prime idea is to export three slightly
lower level functions and thus avoid the need for migrate_vma_ops
callbacks.

Diffstat:

    5 files changed, 281 insertions(+), 607 deletions(-)

A git tree is also available at:

    git://git.infradead.org/users/hch/misc.git migrate_vma-cleanup.2

Gitweb:

    http://git.infradead.org/users/hch/misc.git/shortlog/refs/heads/migrate_vma-cleanup.2

Changes since v1:
 - fix a few whitespace issues
 - drop the patch to remove MIGRATE_PFN_WRITE for now
 - various spelling fixes
 - clear cpages and npages in migrate_vma_setup
 - fix the nouveau_dmem_fault_copy_one return value
 - minor improvements to some nouveau internal calling conventions