rcu: Mark accesses to rcu_state.n_force_qs

commit 2431774f upstream.

This commit marks accesses to the rcu_state.n_force_qs.  These data
races are hard to make happen, but syzkaller was equal to the task.

Reported-by:  <syzbot+e08a83a1940ec3846cd5@syzkaller.appspotmail.com>
Acked-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>