percpu_ref: don't refer to ref->data if it isn't allocated

We can't check ref->data->confirm_switch directly in __percpu_ref_exit(), since
ref->data may not be allocated in one not-initialized refcount.

Fixes: 2b0d3d3e ("percpu_ref: reduce memory footprint of percpu_ref in fast path")
Reported-by: syzbot+fd15ff734dace9e16437@syzkaller.appspotmail.com
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

lib/percpu-refcount.c

@@ -109,7 +109,7 @@ static void __percpu_ref_exit(struct percpu_ref *ref)
 
 	if (percpu_count) {
 		/* non-NULL confirm_switch indicates switching in progress */
-		WARN_ON_ONCE(ref->data->confirm_switch);
+		WARN_ON_ONCE(ref->data && ref->data->confirm_switch);
 		free_percpu(percpu_count);
 		ref->percpu_count_ptr = __PERCPU_REF_ATOMIC_DEAD;
 	}