md/raid10: add rcu protection in raid10_status.

mirrors[].rdev can become NULL at any point unless:
 - a counted reference is held
 - ->reconfig_mutex is held, or
 - rcu_read_lock() is held

raid10_status holds none of these.  So add rcu_read_lock()
protection.
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>

drivers/md/raid10.c

@@ -1495,10 +1495,12 @@ static void raid10_status(struct seq_file *seq, struct mddev *mddev)
 	}
 	seq_printf(seq, " [%d/%d] [", conf->geo.raid_disks,
 					conf->geo.raid_disks - mddev->degraded);
-	for (i = 0; i < conf->geo.raid_disks; i++)
-		seq_printf(seq, "%s",
-			      conf->mirrors[i].rdev &&
-			      test_bit(In_sync, &conf->mirrors[i].rdev->flags) ? "U" : "_");
+	rcu_read_lock();
+	for (i = 0; i < conf->geo.raid_disks; i++) {
+		struct md_rdev *rdev = rcu_dereference(conf->mirrors[i].rdev);
+		seq_printf(seq, "%s", rdev && test_bit(In_sync, &rdev->flags) ? "U" : "_");
+	}
+	rcu_read_unlock();
 	seq_printf(seq, "]");
 }