Manage: fix the mess I made in earlier patch.

Manage: fix the mess I made in earlier patch.

When I separated the 'native metadata' case more cleanly from the
"external metadata" case for adding a drive, I left some 'external'
code in the 'native' case, and didn't copy it to the 'external' case.

When - in the external case - we add to super, we much check for
mdmon first, so we know whether to do the metadata update ourselves
or not, then afterwards call either flush_metadata_updates (to send
to mdmon) or sync_metadata (to do it directly).

Reported-by: Adam Kwolek <adam.kwolek@intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>