dm space map common: remove check for impossible sm_find_free() return value

The function sm_find_free() just returns -ENOSPC and 0.
So remove lone caller's check for some other error.

Signed-off-by: ZhangXiaoxu <zhangxiaoxu5@huawei.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>