deploy: Correctly use libmount unref() calls rather than free()

deploy: Correctly use libmount unref() calls rather than free()

We saw a random ostree SEGV start popping up in our CI environment:
https://github.com/projectatomic/rpm-ostree/pull/641#issuecomment-281870424

Looking at this code more and comparing it to what util-linux does, I noticed we
had a write-after-free, since `mnt_unref_table()` will invoke
`mnt_unref_cache()` on its cache, and that function does:

```
	if (cache) {
		cache->rfcount--;
```

unconditionally.

Fix this by using `unref()`.

Closes: #705
Approved by: jlebon