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 <[e-mail]>
Tested-by: Wanpeng Li <[e-mail]>
Acked-by: Davidlohr Bueso <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Signed-off-by: Paolo Bonzini <[e-mail]>

include/linux/rcuwait.h

···
  */
 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);