df: displayed sizes are in the wrong units

The default units for sizes should be Kilobytes (1024 bytes), but they are shown in bytes.

uutils df:
```
$ ./target/debug/df | sort | head -n1
/dev/loop0           58195968     58195968            0  100% /snap/core18/2253
```

GNU df:
```
$ df | sort | head -n1
/dev/loop0                    56832    56832         0 100% /snap/core18/2253
```

(58195968 / 1024 = 56832)