memcg: initiate deprecation of pressure_level

The pressure_level in memcg v1 provides memory pressure notifications to
the user space.  At the moment it provides notifications for three levels
of memory pressure i.e.  low, medium and critical, which are defined based
on internal memory reclaim implementation details.  More specifically the
ratio of scanned and reclaimed pages during a memory reclaim.  However
this is not robust as there are workloads with mostly unreclaimable user
memory or kernel memory.

For v2, the users can use PSI for memory pressure status of the system or
the cgroup.  Let's start the deprecation process for pressure_level and
add warnings to gather the info on how the current users are using this
interface and how they can be used to PSI.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Shakeel Butt <[email protected]>
Reviewed-by: T.J. Mercier <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Acked-by: Roman Gushchin <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Muchun Song <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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