mm/damon/core: allow quota goals set zero effective size quota

mm/damon/core: allow quota goals set zero effective size quota

User-explicit quotas (size and time quotas) having zero value means the
quotas are unset.  And, effective size quota is set as the minimum value
of the explicit quotas.  When quota goals are set, the goal-based quota
tuner can make it lower.  But the existing only single tuner never sets
the effective size quota zero.  Because of the fact, DAMON core assumes
zero effective quota means the user has set no quota.

Multiple tuners are now allowed, though.  In the future, some tuners might
want to set a zero effective size quota.  There is no reason to restrict
that.  Meanwhile, because of the current implementation, it will only
deactivate all quotas and make the scheme work at its full speed.

Introduce a dedicated function for checking if no quota is set.  The
function checks the fact by showing if the user-set explicit quotas are
zero and no goal is installed.  It is decoupled from zero effective quota,
and hence allows future tuners set zero effective quota for intentionally
deactivating the scheme by a purpose.

Link: https://lkml.kernel.org/r/20260310010529.91162-3-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>