srcu: Check for invalid idx argument in srcu_read_unlock()

The current SRCU implementation has an idx argument of zero or one,
and never anything else.  This commit therefore adds a WARN_ON_ONCE()
to complain if this restriction is violated.
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>

include/linux/srcu.h

@@ -223,6 +223,7 @@ srcu_read_lock_notrace(struct srcu_struct *ssp) __acquires(ssp)
 static inline void srcu_read_unlock(struct srcu_struct *ssp, int idx)
 	__releases(ssp)
 {
+	WARN_ON_ONCE(idx & ~0x1);
 	rcu_lock_release(&(ssp)->dep_map);
 	__srcu_read_unlock(ssp, idx);
 }