6079 add umount when device of mounted filesystem differs

Working on https://github.com/saltstack/salt/issues/6079.. Added logic to umount a mounted filesystem if the mountpoint matches the one requested but the device name doesn't (meaning the mount is being changed). I added a redundant check of whether the filesystem is mounted so that I could umount it and let the existing logic mount it and send back the status codes. I also moved the 'Target was already mounted' message up to my new check because it seemed like a good place for it.