sched: Factor out update_other_load_avgs() from __update_blocked_others()

RT, DL, thermal and irq load and utilization metrics need to be decayed and
updated periodically and before consumption to keep the numbers reasonable.
This is currently done from __update_blocked_others() as a part of the fair
class load balance path. Let's factor it out to update_other_load_avgs().
Pure refactor. No functional changes.

This will be used by the new BPF extensible scheduling class to ensure that
the above metrics are properly maintained.

v2: Refreshed on top of tip:sched/core.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: David Vernet <dvernet@meta.com>