rcu: Apply rcu_seq operations to _rcu_barrier()

The rcu_seq operations were open-coded in _rcu_barrier(), so this commit
replaces the open-coding with the shiny new rcu_seq operations.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 4edc277..5c1042d 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -486,7 +486,7 @@
 	struct mutex barrier_mutex;		/* Guards barrier fields. */
 	atomic_t barrier_cpu_count;		/* # CPUs waiting on. */
 	struct completion barrier_completion;	/* Wake at barrier end. */
-	unsigned long n_barrier_done;		/* ++ at start and end of */
+	unsigned long barrier_sequence;		/* ++ at start and end of */
 						/*  _rcu_barrier(). */
 	/* End of fields guarded by barrier_mutex. */