mount: use blkid to determine fstype

* Add a function to volume_id that returns an fstype for a
  given device.

* During mount, if the fstype is auto, or blkid disagrees
  with the given fstype, trust blkid's determination of the
  filesystem type and run with it.

Change-Id: I357cbb5d255a30a27152a06de5328b2ef14553f9
3 files changed