memcg: fix fatal livelock in kswapd

[James Bottomley]

The fatal livelock in kswapd, reported in this thread:

http://marc.info/?t=130392066000001

Is mitigateable if we prevent the cgroups code being so aggressive in
its zone shrinking (by reducing it's default shrink from 0 [everything]
to DEF_PRIORITY [some things]).  This will have an obvious knock on
effect to cgroup accounting, but it's better than hanging systems.

Signed-off-by: James Bottomley <James.Bottomley@xxxxxxx>

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