fix examine_brief segfault

When performing an "-Ebs -e <metadata type>" we segfault because the
superblock has been freed too early.  We also leak memory for 'ddf' and
'imsm' because, unlike super[01], we do not implicitly free when
->load_super is called on an already loaded supertype.

So, fix up imsm and ddf to match type 0 and 1 ->load_super() semantics,
and update Examine to not free the superblock until all usages have been
exhausted.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>