mm/damon/sysfs-schemes: implement DAMOS action destinations directory

DAMOS_MIGRATE_{HOT,COLD} can have multiple action destinations and their
weights.  Implement sysfs directory named 'dests' under each scheme
directory to let DAMON sysfs ABI users utilize the feature.  The interface
is similar to other multiple parameters directory like kdamonds or
filters.  The directory contains only nr_dests file initially.  Writing a
number of desired destinations to nr_dests creates directories of the
number.  Each of the created directories has two files named id and
weight.  Users can then write the destination's identifier (node id in
case of DAMOS_MIGRATE_*) and weight to the files.

Link: https://lkml.kernel.org/r/20250709005952.17776-4-bijan311@gmail.com


Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Bijan Tabatabai <bijantabatab@micron.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Ravi Shankar Jonnalagadda <ravis.opensrc@micron.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>