cg: Don't walk cgroup tree when it's not required

We have two bugs actually.

First, the check for 'item == root_item' in dump_task_cgroup fires
twice: first when we rite inventory (item == NULL as argument and
root_item == NULL because we haven't yet collected tasks) and the
2nd time when we dump the root task itself.

The 2nd issue sits in dump_cgroups() -- if root_cgset == criu_cgset
we don't write cgroups information at all (checking that we don't
have them with list_is_singular() inside that if). That said, we
don't need to read the cgroups tree if we're not going to dump it.

This patch fixes both.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Acked-by: Andrew Vagin <avagin@parallels.com>
Acked-by: Tycho Andersen <tycho.andersen@canonical.com>