mm preparatory patches for HMM and IOMMUv2

["Jérôme Glisse" <j.glisse@gmail.com>]

Andrew so here are a set of mm patch that do some ground modification to core
mm code. They apply on top of today's linux-next and they pass checkpatch.pl
with flying color (except patch 4 but i did not wanted to be a nazi about 80
char line).

Patch 1 is the mmput notifier call chain we discussed with AMD.

Patch 2, 3 and 4 are so far only useful to HMM but i am discussing with AMD and
i believe it will be useful to them to (in the context of IOMMUv2).

Patch 2 allows to differentiate page unmap for vmscan reason or for poisoning.

Patch 3 associate mmu_notifier with an event type allowing to take different code
path inside mmu_notifier callback depending on what is currently happening to the
cpu page table. There is no functional change, it just add a new argument to the
various mmu_notifier calls and callback.

Patch 4 pass along the vma into which the range invalidation is happening. There
is few functional changes in place where mmu_notifier_range_invalidate_start/end
used [0, -1] as range, instead now those place call the notifier once for each
vma. This might prove to add unwanted overhead hence why i did it as a separate
patch.

I did not include the core hmm patch but i intend to send a v4 next week. So i
really would like to see those included for next release.

As usual comments welcome.

Cheers,
Jérôme Glisse