rcu: Fix exp_funnel_lock()/rcu_exp_wait_wake() datarace

The rcu_node structure's ->exp_seq_rq field is accessed locklessly, so
updates must use WRITE_ONCE().  This commit therefore adds the needed
WRITE_ONCE() invocation where it was missed.

This data race was reported by KCSAN.  Not appropriate for backporting
due to failure being unlikely.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>