lsns: reorganize members specifying other namespaces in lsns_namespace

lsns: reorganize members specifying other namespaces in lsns_namespace

This is a preparation change for planed "tree interpolation" patch.

parentid, ownerid, parentns, and ownerns were members of
lsns_namespace struct. When interpolating missing namespaces
for building a unified name space tree, duplicated functions
are needed; one is for parentid and parentns members, and
another is for ownerid and ownerns members. To avoid the
duplication, this change unifies the members as following:

 parentid and ownerid => related_id [2]
 parentns and ownerns => related_ns [2]

Signed-off-by: Masatake YAMATO <yamato@redhat.com>