dm space map disk: fix some book keeping in the disk space map

commit 0377a07c upstream.

When decrementing the reference count for a block, the free count wasn't
being updated if the reference count went to zero.

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>