dm writecache: advance the number of arguments when reporting max_age

When reporting the "max_age" value the number of arguments must
advance by two.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Fixes: 3923d485 ("dm writecache: implement gradual cleanup")
Cc: stable@vger.kernel.org # v5.7+
Signed-off-by: Mike Snitzer <snitzer@redhat.com>