mm/damon/sysfs-schemes: implement quotas->goal_tuner file

mm/damon/sysfs-schemes: implement quotas->goal_tuner file

Add a new DAMON sysfs interface file, namely 'goal_tuner' under the DAMOS
quotas directory.  It is connected to the damos_quota->goal_tuner field.
Users can therefore select their favorite goal-based quotas tuning
algorithm by writing the name of the tuner to the file.  Reading the file
returns the name of the currently selected tuner.

Link: https://lkml.kernel.org/r/20260310010529.91162-5-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>