sched/fair: Use helper functions to access root_domain::overload

Introduce two helper functions to access & set the root_domain::overload flag:

  get_rd_overload()
  set_rd_overload()

To make sure code is always following READ_ONCE()/WRITE_ONCE() access methods.

No change in functionality intended.

[ mingo: Renamed the accessors to get_/set_rd_overload(), tidied up the changelog. ]

Suggested-by: Qais Yousef <qyousef@layalina.io>
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/20240325054505.201995-3-sshegde@linux.ibm.com