LVM: report thin pool capacity in a reasonable way

Now that thin pools are created of adequate size, capacity reporting
must be updated so that users can successfully create new volumes.  The
existing code reports the size and remaining capacity of the volume
group.  When 'thin' is enabled, the thin pool takes up nearly all of the
volume group and available capacity appears to be 0 - preventing any new
volumes from being created.

This patch changes thin LVM capacity reporting to always report
available capacity as the total size of the thin pool.  For users, this
means a couple of things:

  1. No single volume can exceed the size of the thin pool.
  2. An infinite number of volumes can be created, all backed by the
     same thin pool.

This allows infinite over-provisioning of thin LVM.  It is left to the
user to monitor the actual free space remaining in the pool and make use
of thin volumes accordingly.

Additional improvements are being considered, so expect updates to this
logic in the near future.

Change-Id: I8a2b8ce972cb0a585555dd92c78dd8bbade1a9b2
Closes-Bug: #1249782