mm, memcg: Prevent memory.swappiness load/store tearing

The knob for cgroup v1 memory controller: memory.swappiness is not
protected by any locking so it can be modified while it is used.  This is
not an actual problem because races are unlikely.  But it is better to use
[READ|WRITE]_ONCE to prevent compiler from doing anything funky.

The access of memcg->swappiness and vm_swappiness is lockless, so both of
them can be concurrently set at the same time as we are trying to read
them.  All occurrences of memcg->swappiness and vm_swappiness are updated
with [READ|WRITE]_ONCE.

[[email protected]: v3]
  Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Yue Zhao <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Acked-by: Shakeel Butt <[email protected]>
Acked-by: Roman Gushchin <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: Tang Yizhou <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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