mm/vmstat: annotate data race for zone->free_area[order].nr_free

KCSAN reports a data-race on v5.10 which also exists on mainline:

  BUG: KCSAN: data-race in extfrag_for_order+0x33/0x2d0

  race at unknown origin, with read to 0xffff9ee9bfffab48 of 8 bytes by task 34 on cpu 1:
   extfrag_for_order+0x33/0x2d0
   kcompactd+0x5f0/0xce0
   kthread+0x1f9/0x220
   ret_from_fork+0x22/0x30

  Reported by Kernel Concurrency Sanitizer on:
  CPU: 1 PID: 34 Comm: kcompactd0 Not tainted 5.10.0+ #2
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Ubuntu-1.8.2-1ubuntu1 04/01/2014

Access to zone->free_area[order].nr_free in extfrag_for_order() and
frag_show_print() is lockless.  That's intentional and the stats are a
rough estimate anyway.  Annotate them with data_race().

[liushixin2@huawei.com: add comments]
  Link: https://lkml.kernel.org/r/20210918084655.2696522-1-liushixin2@huawei.com

Link: https://lkml.kernel.org/r/20210908015606.3999871-1-liushixin2@huawei.com


Signed-off-by: Liu Shixin <liushixin2@huawei.com>
Cc: "Paul E . McKenney" <paulmck@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>