sched_ext: Use READ_ONCE() for lock-free reads of module param variables

bypass_lb_cpu() reads scx_bypass_lb_intv_us and scx_slice_bypass_us
without holding any lock, in timer callback context where module
parameter writes via sysfs can happen concurrently:

    min_delta_us = scx_bypass_lb_intv_us / SCX_BYPASS_LB_MIN_DELTA_DIV;
                   ^^^^^^^^^^^^^^^^^^^^
                   plain read -- KCSAN data race

    if (delta < DIV_ROUND_UP(min_delta_us, scx_slice_bypass_us))
                                           ^^^^^^^^^^^^^^^^^
                                           plain read -- KCSAN data race

scx_bypass_lb_intv_us already uses READ_ONCE() in scx_bypass_lb_timerfn()
and scx_bypass() for its other lock-free read sites, leaving
bypass_lb_cpu() inconsistent. scx_slice_bypass_us has the same
lock-free access pattern in the same function.

Fix both plain reads by using READ_ONCE() to complete the concurrent
access annotation and make the code KCSAN-clean.

Signed-off-by: zhidao su <suzhidao@xiaomi.com>
Signed-off-by: Tejun Heo <tj@kernel.org>