df: df doesn't check if the input path is a filesystem

[crazystylus]

For example when we pass /dev/sda3 to df, it should print details of /dev/sda3 instead of considering it as mount point/filesystem path. Below are some example outputs:
gnu df

$ df --out=source,target /dev/sda3 .
Filesystem     Mounted on
/dev/sda3      /root
/dev/sda3      /root

uutils df

../../../target/release/df --out=source,target /dev/sda3 .
Filesystem       Mounted on
udev             /dev
/dev/sda3      /root