fork: annotate data race in copy_process()

KCSAN reported data race reading and writing nr_threads and max_threads.
The data race is intentional and benign. This is obvious from the comment
above it and based on general consensus when discussing this issue. So
there's no need for any heavy atomic or *_ONCE() machinery here.

In accordance with the newly introduced data_race() annotation consensus,
mark the offending line with data_race(). Here it's actually useful not
just to silence KCSAN but to also clearly communicate that the race is
intentional. This is especially helpful since nr_threads is otherwise
protected by tasklist_lock.

BUG: KCSAN: data-race in copy_process / copy_process

write to 0xffffffff86205cf8 of 4 bytes by task 14779 on cpu 1:
  copy_process+0x2eba/0x3c40 kernel/fork.c:2273
  _do_fork+0xfe/0x7a0 kernel/fork.c:2421
  __do_sys_clone kernel/fork.c:2576 [inline]
  __se_sys_clone kernel/fork.c:2557 [inline]
  __x64_sys_clone+0x130/0x170 kernel/fork.c:2557
  do_syscall_64+0xcc/0x3a0 arch/x86/entry/common.c:294
  entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffffffff86205cf8 of 4 bytes by task 6944 on cpu 0:
  copy_process+0x94d/0x3c40 kernel/fork.c:1954
  _do_fork+0xfe/0x7a0 kernel/fork.c:2421
  __do_sys_clone kernel/fork.c:2576 [inline]
  __se_sys_clone kernel/fork.c:2557 [inline]
  __x64_sys_clone+0x130/0x170 kernel/fork.c:2557
  do_syscall_64+0xcc/0x3a0 arch/x86/entry/common.c:294
  entry_SYSCALL_64_after_hwframe+0x44/0xa9
Link: https://groups.google.com/forum/#!msg/syzkaller-upstream-mo


deration/thvp7AHs5Ew/aPdYLXfYBQAJ

Reported-by:  <syzbot+52fced2d288f8ecd2b20@syzkaller.appspotmail.com>
Signed-off-by: Zefan Li <lizefan@huawei.com>
Signed-off-by: Weilong Chen <chenweilong@huawei.com>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Cc: Qian Cai <cai@lca.pw>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Christian Brauner <christian.brauner@ubuntu.com>
Cc: Marco Elver <elver@google.com>
[christian.brauner@ubuntu.com: rewrite commit message]
Link: https://lore.kernel.org/r/20200623041240.154294-1-chenweilong@huawei.com


Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>