Gets the root of the mount within the filesystem. This is useful e.g. for mounts created by bind operation, or btrfs subvolumes.
For example, the root path is equal to “/” for mount created by “mount /dev/sda1 /mnt/foo” and “/bar” for “mount —bind /mnt/foo/bar /mnt/bar”.
Available since: 2.60
The data is owned by the caller of the function.