Fix new incorrect error return from do_md_stop.

Recent commit c8c00a6915a2e3d10416e8bdd3138429beb96210
changed the exit paths in do_md_stop and was not quite
careful enough.  There is one path were 'err' now needs
to be cleared but it isn't.
So setting an array to readonly (with mdadm --readonly) will
work, but will incorrectly report and error: ENXIO.

Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/md.c

@@ -4364,6 +4364,7 @@ static int do_md_stop(mddev_t * mddev, int mode, int is_open)
 		if (mode == 1)
 			set_disk_ro(disk, 1);
 		clear_bit(MD_RECOVERY_FROZEN, &mddev->recovery);
+		err = 0;
 	}
 out:
 	mutex_unlock(&mddev->open_mutex);