md/raid5: add rcu protection to rdev accesses in raid5_status.

	seq_printf(seq, " level %d, %dk chunk, algorithm %d", mddev->level,

		conf->chunk_sectors / 2, mddev->layout);

	seq_printf (seq, " [%d/%d] [", conf->raid_disks, conf->raid_disks - mddev->degraded);

	for (i = 0; i < conf->raid_disks; i++)

		seq_printf (seq, "%s",

			       conf->disks[i].rdev &&

			       test_bit(In_sync, &conf->disks[i].rdev->flags) ? "U" : "_");

	rcu_read_lock();

	for (i = 0; i < conf->raid_disks; i++) {

		struct md_rdev *rdev = rcu_dereference(conf->disks[i].rdev);

		seq_printf (seq, "%s", rdev && test_bit(In_sync, &rdev->flags) ? "U" : "_");

	}

	rcu_read_unlock();

	seq_printf (seq, "]");

}