sched_ext: Use update_curr_common() in update_curr_scx()

update_curr_scx() is open coding runtime updates. Use update_curr_common()
instead and avoid unnecessary deviations.

Signed-off-by: Tejun Heo <tj@kernel.org>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Acked-by: David Vernet <void@manifault.com>