Cpuset hardwall flag: Introduction

Currently the cpusets mem_exclusive flag is overloaded to mean both
"no-overlapping" and "no GFP_KERNEL allocations outside this cpuset".

These patches add a new mem_hardwall flag with just the allocation
restriction part of the mem_exclusive semantics, without breaking
backwards-compatibility for those who continue to use just
mem_exclusive. Additionally, the cgroup control file registration for
cpusets is cleaned up to reduce boilerplate.

Signed-off-by: Paul Menage <menage@google.com>

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