ceph-disk: dereference symlinks in destroy and zap

ceph-disk: dereference symlinks in destroy and zap

The behavior of partprobe or sgdisk may be subtly different if given a
symbolic link to a device instead of an actual device. The debug output
is also more confusing when the symlink shows instead of the device it
points to.

Always dereference the symlink before running destroy and zap.

Signed-off-by: Loic Dachary <loic@dachary.org>
(cherry picked from commit fe71647bc9bd0f9ddc6d470ee7bee1e6b0983e2b)

Conflicts:
        src/ceph-disk
          trivial, because destroy is not implemented
          in infernalis