[PATCH stable-only] mm: fix classzone_idx underflow in shrink_zones()

[Vlastimil Babka]

Hi,

I realize this is against the standard stable policy, but I see no other
way, because the mainline accidental fix is part of 34+ patch reclaim
rework, that would be absurd to try to backport into stable. The fix is
a one-liner though.

The bug affects at least 4.4.y, and likely also older stable trees that
backported commit 7bf52fb891b6, which itself was a fix for 3.19 commit
6b4f7799c6a5. You could revert the 7bf52fb891b6 backport, but then 32bit
with highmem might suffer from OOM or thrashing.

More details in the changelog itself.

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