dev_pagemap related cleanups v2

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

Git tree:

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

Gitweb:

    http://git.infradead.org/users/hch/misc.git/shortlog/refs/heads/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