cleanup the walk_page_range interface v2

[Christoph Hellwig]

Hi all,

this series is based on a patch from Linus to split the callbacks
passed to walk_page_range and walk_page_vma into a separate structure
that can be marked const, with various cleanups from me on top.

This series is also available as a git tre here:

    git://git.infradead.org/users/hch/misc.git pagewalk-cleanup

Gitweb:

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


Diffstat:

    14 files changed, 291 insertions(+), 273 deletions(-)

Changes since v1:
 - minor comment typo and checkpatch fixes
 - fix a compile failure for !CONFIG_SHMEM
 - rebased to the wip/jgg-hmm branch