Add function that uses df to  determine the mountpoint of a device
Add function that uses df to  determine the mountpoint of a device

Will be used in future PR to get the XFS stats for a filesystem.

Signed-off-by: Todd Gill <7afbb5ba98932b8ffb9cc9e6675c4792db142529@redhat.com>

trgill/stratisd,stratis-storage/stratisd,trgill/stratisd,stratis-storage/stratisd,mulkieran/stratisd,stratis-storage/stratisd,mulkieran/stratisd