dev_pagemap related cleanups v2

From: Christoph Hellwig
Date: Mon Jun 17 2019 - 08:32:41 EST

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 the rdma/hmm branch + the dev_pagemap
releas fix series from Dan that went into 5.2-rc5.

Git tree:

git:// hmm-devmem-cleanup.2


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