sched/cache: Annotate lockless accesses to mm->sc_stat.cpu

mm->sc_stat.cpu is written by task_cache_work() and could be read
locklessly by several functions on other CPUs.  Use READ_ONCE and
WRITE_ONCE on mm->sc_stat.cpu access and write to prevent inconsistent
values from compiler optimizations when there are multiple accesses.

For example in get_pref_llc(), if the writer updated the field between
two compiler-generated loads, the validation (e.g., cpu != -1) and
subsequent use (e.g., llc_id(cpu)) could operate on different values,
allowing a negative CPU ID to be used as an index.

Leave plain write in mm_init_sched(), where the mm is not
yet visible to other CPUs.

This bug was reported by sashiko.

Fixes: 47d8696b95f7 ("sched/cache: Assign preferred LLC ID to processes")
Signed-off-by: Chen Yu <yu.c.chen@intel.com>
Co-developed-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/63ea494f12efcf265d7134400a06cd75d7f2c310.1778703694.git.tim.c.chen@linux.intel.com

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