dm space map disk: remove redundant calls to sm_disk_get_nr_free()

Both sm_disk_new_block and sm_disk_commit are needlessly calling
sm_disk_get_nr_free(). Looks like old queries used for some
debugging.

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>