DDF: brief_detail_super_ddf: print correct UUID for subarrays

DDF: brief_detail_super_ddf: print correct UUID for subarrays

Commit c1ea5a98 caused brief_detail_super_ddf() to be called
for subarrays. But the UUID printed was always the one of the
container. This is wrong and actually worse than printing no UUID
at all, and causes the DDF test case (10ddf-create) to fail.

This patch adds code to determine the MD UUID of a subarray correctly.
The hard part is to figure out for which subarray the function is
called. Moved that to an extra function.

Signed-off-by: Martin Wilck <mwilck@arcor.de>
Signed-off-by: NeilBrown <neilb@suse.de>