sched/fair: Introduce is_rd_overutilized() helper function to access root_domain::overutilized

The root_domain::overutilized field is READ_ONCE() accessed in
multiple places, which could be simplified with a helper function.

This might also make it more apparent that it needs to be used
only in case of EAS.

No change in functionality intended.

Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Qais Yousef <qyousef@layalina.io>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://lore.kernel.org/r/20240307085725.444486-3-sshegde@linux.ibm.com