sched/core: uclamp: Map TG's clamp values into CPU's clamp buckets

Utilization clamping requires to map each different clamp value into one
of the available clamp buckets used at {en,de}queue time (fast-path).
Each time a TG's clamp value sysfs attribute is updated via:
   cpu_util_{min,max}_write_u64()
we need to update the task group reference to the new value's clamp
bucket and release the reference to the previous one.

Ensure that, whenever a task group is assigned a specific clamp_value,
this is properly translated into a unique clamp bucket to be used in the
fast-path. Do it by slightly refactoring uclamp_bucket_inc() to make the
(*task_struct) parameter optional and by reusing the code already
available for the per-task API.

Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Tejun Heo <tj@kernel.org>

---
Changes in v6:
 Others:
 - wholesale s/group/bucket/
 - wholesale s/_{get,put}/_{inc,dec}/ to match refcount APIs