sched/cache: Decouple sched_cache_group from mm to fix UAF

Currently the sched cache grouping is by mm and the scheduling statistics
sched_cache_stat lives in the mm structure.  This ties the life cycle
of scheduling stats with mm.

In account_mm_sched(), the scheduling stats are accessed by
task->mm->sc_stat.  However, a task may be switching mm on one CPU when
another CPU is running account_mm_sched(), and possibly accessing the
old mm that was freed.  This problem was found when running tests with
KASAN by Hyunwoo:

  https://lore.kernel.org/lkml/apPb-Dr4nPYuHQOK@v4bel/

Instead of serializing the mm access by introducing extra acquisition of
rq lock in the mm free path, extract sched_cache_stat from mm_struct,
rename it as sched_cache_group and manage its life cycle apart from
mm_struct with its own ref counting.  This allows us in the next patch
access sched_cache_group directly from task, and add a refcount
on sched_cache_group when a task links to it. This prevents the use
after free issue when accessing stale and released old mm and its
sched cache stat a task switches to a new mm while account_mm_sched()
is done elsewhere.

The other benefit of this restructure is in the future, the grouping of
tasks to a LLC would have the flexibility to be associated with a user
defined grouping, or cgroup, cookie group, numa_group or others instead
of just with a single mm address space.

Rename sched_cache_stat to sched_cache_group and turn it into a refcounted
object allocated from mm_struct.  The mm_struct now holds a pointer
(sched_cache_grp) to this object instead of embedding it.

Fixes: df0d98475954 ("sched/cache: Introduce infrastructure for cache-aware load balancing")
Closes: https://lore.kernel.org/lkml/apPb-Dr4nPYuHQOK@v4bel/
Closes: https://lore.kernel.org/all/343a7e07-7fad-4979-9c9b-82ec038c293c@linux.dev/
Reported-by: Hyunwoo Kim <imv4bel@gmail.com>
Reported-by: Zenghui Yu (Huawei) <zenghui.yu@linux.dev>
Co-developed-by: Chen Yu <yu.c.chen@intel.com>
Signed-off-by: Chen Yu <yu.c.chen@intel.com>
Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: <stable@kernel.org> #7.2.x
Link: https://patch.msgid.link/91fd1e3266707c865bc9abecfb3e17bc676712df.1790035273.git.tim.c.chen@linux.intel.com

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