mm/swap: Do not abuse the seqcount_t latching API

Commit eef1a429f234 ("mm/swap.c: piggyback lru_add_drain_all() calls")
implemented an optimization mechanism to exit the to-be-started LRU
drain operation (name it A) if another drain operation *started and
finished* while (A) was blocked on the LRU draining mutex.

This was done through a seqcount_t latch, which is an abuse of its
semantics:

  1. seqcount_t latching should be used for the purpose of switching
     between two storage places with sequence protection to allow
     interruptible, preemptible, writer sections. The referenced
     optimization mechanism has absolutely nothing to do with that.

  2. The used raw_write_seqcount_latch() has two SMP write memory
     barriers to insure one consistent storage place out of the two
     storage places available. A full memory barrier is required
     instead: to guarantee that the pagevec counter stores visible by
     local CPU are visible to other CPUs -- before loading the current
     drain generation.

Beside the seqcount_t API abuse, the semantics of a latch sequence
counter was force-fitted into the referenced optimization. What was
meant is to track "generations" of LRU draining operations, where
"global lru draining generation = x" implies that all generations
0 < n <= x are already *scheduled* for draining -- thus nothing needs
to be done if the current generation number n <= x.

Remove the conceptually-inappropriate seqcount_t latch usage. Manually
implement the referenced optimization using a counter and SMP memory
barriers.

Note, while at it, use the non-atomic variant of cpumask_set_cpu(),
__cpumask_set_cpu(), due to the already existing mutex protection.

Signed-off-by: Ahmed S. Darwish <a.darwish@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/87y2pg9erj.fsf@vostro.fn.ogness.net

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