srcu: Make grace-period determination use ssp->srcu_reader_flavor

This commit causes the srcu_readers_unlock_idx() function to take the
srcu_struct structure's ->srcu_reader_flavor field into account.  This
ensures that structures defined via DEFINE_SRCU_FAST( or initialized via
init_srcu_struct_fast() have their grace periods use synchronize_srcu()
or synchronize_srcu_expedited() instead of smp_mb(), even before the
first SRCU reader has been entered.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: <bpf@vger.kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>

Author: paulmckrcu

kernel/rcu/srcutree.c

@@ -490,7 +490,7 @@ static bool srcu_readers_lock_idx(struct srcu_struct *ssp, int idx, bool gp, uns
 static unsigned long srcu_readers_unlock_idx(struct srcu_struct *ssp, int idx, unsigned long *rdm)
 {
 	int cpu;
-	unsigned long mask = 0;
+	unsigned long mask = ssp->srcu_reader_flavor;
 	unsigned long sum = 0;
 
 	for_each_possible_cpu(cpu) {