dm: free io_barrier after blk_cleanup_queue call

dm_old_request_fn() has paths that access md->io_barrier.  The party
destroying io_barrier should ensure that no future execution of
dm_old_request_fn() is possible.  Move io_barrier destruction to below
blk_cleanup_queue() to ensure this and avoid a NULL pointer crash during
request-based DM device shutdown.

Cc: stable@vger.kernel.org # 4.3+
Signed-off-by: Tahsin Erdogan <tahsin@google.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>