mount: simplify mnt_build_ids_tree()

This patch reworks this function, removes mess and fix
the next coverity issue:

394                m->parent = parent;
CID 175314 (#1 of 1): Dereference after null check (FORWARD_NULL)
27. var_deref_model: Passing null pointer &parent->children to list_add_tail, which dereferences it. [show details]

travis-ci: success for mount: simplify mnt_build_ids_tree()
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>