dm thin: disable discard support for thin devices if pool's is disabled

If the pool is configured with 'ignore_discard' its discard support is
disabled.  The pool's thin devices should also have queue_limits that
reflect discards are disabled.

Fixes: 34fbcf62 ("dm thin: range discard support")
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Cc: stable@vger.kernel.org # 4.1+