sched_ext: Use READ_ONCE() for plain reads of scx_watchdog_timeout

scx_watchdog_timeout is written with WRITE_ONCE() in scx_enable():

WRITE_ONCE(scx_watchdog_timeout, timeout);

However, three read-side accesses use plain reads without the matching
READ_ONCE():

/* check_rq_for_timeouts() - L2824 */
last_runnable + scx_watchdog_timeout

/* scx_watchdog_workfn() - L2852 */
scx_watchdog_timeout / 2

/* scx_enable() - L5179 */
scx_watchdog_timeout / 2

The KCSAN documentation requires that if one accessor uses WRITE_ONCE()
to annotate lock-free access, all other accesses must also use the
appropriate accessor. Plain reads alongside WRITE_ONCE() leave the pair
incomplete and can trigger KCSAN warnings.

Note that scx_tick() already uses the correct READ_ONCE() annotation:

last_check + READ_ONCE(scx_watchdog_timeout)

Fix the three remaining plain reads to match, making all accesses to
scx_watchdog_timeout consistently annotated and KCSAN-clean.

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