rcuwait: avoid lockdep splats from rcuwait_active()

rcuwait_active only returns whether w->task is not NULL.  This is
exactly one of the usecases that are mentioned in the documentation
for rcu_access_pointer() where it is correct to bypass lockdep checks.

This avoids a splat from kvm_vcpu_on_spin().
Reported-by: Wanpeng Li <kernellwp@gmail.com>
Tested-by: Wanpeng Li <kernellwp@gmail.com>
Acked-by: Davidlohr Bueso <dave@stgolabs.net>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

include/linux/rcuwait.h

@@ -31,7 +31,7 @@ static inline void rcuwait_init(struct rcuwait *w)
  */
 static inline int rcuwait_active(struct rcuwait *w)
 {
-	return !!rcu_dereference(w->task);
+	return !!rcu_access_pointer(w->task);
 }
 
 extern int rcuwait_wake_up(struct rcuwait *w);