dev_pagemap related cleanups v3

[Christoph Hellwig]

Hi Dan, JÃrÃme and Jason,

below is a series that cleans up the dev_pagemap interface so that
it is more easily usable, which removes the need to wrap it in hmm
and thus allowing to kill a lot of code

Note: this series is on top of Linux 5.2-rc5 and has some minor
conflicts with the hmm tree that are easy to resolve.

Diffstat summary:

 32 files changed, 361 insertions(+), 1012 deletions(-)

Git tree:

    git://git.infradead.org/users/hch/misc.git hmm-devmem-cleanup.3

Gitweb:

    http://git.infradead.org/users/hch/misc.git/shortlog/refs/heads/hmm-devmem-cleanup.3


Changes since v2:
 - fix nvdimm kunit build
 - add a new memory type for device dax
 - fix a few issues in intermediate patches that didn't show up in the end
   result
 - incorporate feedback from Michal Hocko, including killing of
   the DEVICE_PUBLIC memory type entirely

Changes since v1:
 - rebase
 - also switch p2pdma to the internal refcount
 - add type checking for pgmap->type
 - rename the migrate method to migrate_to_ram
 - cleanup the altmap_valid flag
 - various tidbits from the reviews