mm: restart if multiple traversals raced

Currently, if multiple reclaimers raced on the same position, the
reclaimers which detect the race will still reclaim from the same memcg. 
Instead, the reclaimers which detect the race should move on to the next
memcg in the hierarchy.

So, in the case where multiple traversals race, jump back to the start of
the mem_cgroup_iter() function to find the next memcg in the hierarchy to
reclaim from.

Link: https://lkml.kernel.org/r/[email protected]
Reported-by: [email protected]
Closes: https://lore.kernel.org/[email protected]/
Signed-off-by: Kinsey Ho <[email protected]>
Reviewed-by: T.J. Mercier <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Michal Koutný <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: Roman Gushchin <[email protected]>
Cc: Shakeel Butt <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: Yosry Ahmed <[email protected]>
Cc: Zefan Li <[email protected]>
Cc: Hugh Dickins <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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