[Artful] (upstream) cherry-pick mmu_notifier fix to prevent Windows BSODs

[Fabian Grünbichler]

BugLink: https://bugs.launchpad.net/bugs/1738972

Impact: Windows 2016 and 10 VMs running in Qemu VMs using KVM trigger
BSODs under certain memory conditions

Fix: Single follow-up upstream cherry pick which fixes the problem.

Regression Potential: Merged in 4.15 and 4.14.6, tested by multiple
affected users.

see https://marc.info/?l=linux-kernel&m=151203441531472&w=2 for the long
writeup

clean cherry-pick from 4.14.6
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