Currently we rely on the fact that ->mntinfo_list was already
collected by walk_namespaces(walk_all => true), but we are going
to change this.

This patch simply adds collect_mntinfo(ns) into find_ext_ns_id() if
->mntinfo_list == NULL. This is all we need for this ns_id if it was
not initialized by collect_mnt_namespaces().
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Tested-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>