dm thin: fix missing pool reference count decrement in pool_ctr error path

commit ba30670f4d5292c4e7f7980bbd5071f7c4794cdd upstream.

Fixes: ac8c3f3d ("dm thin: generate event when metadata threshold passed")
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>