sched/cache: Introduce task_struct->sched_cache_grp to fix UAF

Add a sched_cache_grp pointer to task_struct so that scheduler code
can access the cache group directly via the task, without going
through mm->sched_cache_grp.  This decouples the scheduler's hot-path
accesses from the mm_struct.

Each task holds its own refcount on the sched_cache_group, separate
from the reference held by its mm_struct.  The reference is acquired
in copy_mm() (fork) and exec_mmap() (exec), and released in exit_mm().
This fixes the use-after-free when account_mm_sched() reaches the group
through a task whose mm is being switched, as reported by Hyunwoo:

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

Convert all scheduler code in fair.c and exit.c to use
p->sched_cache_grp instead of p->mm->sched_cache_grp.

Keep the fork/exec/exit reference management out of the generic mm
paths: add sched_cache_fork(), sched_cache_fork_cleanup(),
sched_cache_exec_mmap() and sched_cache_exit_mm() in
kernel/sched/cache_sched.c (with empty stubs for !CONFIG_SCHED_CACHE),
so fs/exec.c, kernel/fork.c and kernel/exit.c each call one helper
instead of open-coding the refcounting under #ifdef.  Also add
sched_cache_group_get() and task_cache_group_get().

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/ae7081dc54736bf115215f9867abb2711a7403fb.1790035273.git.tim.c.chen@linux.intel.com

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