mm: Add transformation message for per-memcg swappiness

The concept of per-memcg swappiness has never landed well in memcg for
cgroup v2. Add a message to users who use it on v1 hierarchy.
Decreased swappiness transforms to memory.swap.max=0 whereas
increased swappiness transforms into active memory.reclaim operation.

Link: https://lore.kernel.org/r/1577252208-32419-1-git-send-email-teawater@gmail.com/


Signed-off-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Tejun Heo <tj@kernel.org>