mm: vmpressure: don't count proactive reclaim in vmpressure

memory.reclaim is a cgroup v2 interface that allows users to proactively
reclaim memory from a memcg, without real memory pressure.  Reclaim
operations invoke vmpressure, which is used: (a) To notify userspace of
reclaim efficiency in cgroup v1, and (b) As a signal for a memcg being
under memory pressure for networking (see
mem_cgroup_under_socket_pressure()).

For (a), vmpressure notifications in v1 are not affected by this change
since memory.reclaim is a v2 feature.

For (b), the effects of the vmpressure signal (according to Shakeel [1])
are as follows:
1. Reducing send and receive buffers of the current socket.
2. May drop packets on the rx path.
3. May throttle current thread on the tx path.

Since proactive reclaim is invoked directly by userspace, not by memory
pressure, it makes sense not to throttle networking.  Hence, this change
makes sure that proactive reclaim caused by memory.reclaim does not
trigger vmpressure.

[1] https://lore.kernel.org/lkml/CALvZod68WdrXEmBpOkadhB5GPYmCXaDZzXH=yyGOCAjFRn4NDQ@mail.gmail.com/

[[email protected]: update documentation]
  Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Yosry Ahmed <[email protected]>
Acked-by: Shakeel Butt <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Acked-by: David Rientjes <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Roman Gushchin <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Miaohe Lin <[email protected]>
Cc: NeilBrown <[email protected]>
Cc: Alistair Popple <[email protected]>
Cc: Suren Baghdasaryan <[email protected]>
Cc: Peter Xu <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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