md/raid5: don't test ->writes_pending in raid5_remove_disk

This test on ->writes_pending cannot be safe as the counter
can be incremented at any moment and cannot be locked against.

Change it to test conf->active_stripes, which at least
can be locked against.  More changes are still needed.

A future patch will change ->writes_pending, and testing it here will
be very inconvenient.
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>

drivers/md/raid5.c

@@ -7532,9 +7532,12 @@ static int raid5_remove_disk(struct mddev *mddev, struct md_rdev *rdev)
 		/*
 		 * we can't wait pending write here, as this is called in
 		 * raid5d, wait will deadlock.
+		 * neilb: there is no locking about new writes here,
+		 * so this cannot be safe.
 		 */
-		if (atomic_read(&mddev->writes_pending))
+		if (atomic_read(&conf->active_stripes)) {
 			return -EBUSY;
+		}
 		log_exit(conf);
 		return 0;
 	}