Revert "Revert "sched: Rework pick_next_task() slow-path""

This reverts commit 9acb7c07.

The root problem has now been fixed in 5.4-rc7, so revert this to
prevent merge issues.

Bug: 142182814
Cc: Quentin Perret <qperret@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>