ip vrf: Handle VRF nesting in namespace

ip vrf: Handle VRF nesting in namespace

Since cgroups are not namespace aware, the directory heirarchy used by
ip vrf should account for network namespaces. In this case, change the
path from CGRP/BASE/vrf/NAME to CGRP/BASE/NETNS/vrf/NAME where CGRP is
the cgroup2 mount path, BASE in any base heirarchy inherited before VRF
is applied and NAME is the VRF name.

The intent is as follows: a user logs into the box into some namespace
with a name known to iproute2. Some other policy may have put the
process into a BASE heirarchy. From there the user executes a task in
a VRF and in doing so the task heirarchy becomes CGRP/BASE/NETNS/vrf/NAME.
The namespace level is omitted for the default namespace.

Reported-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: David Ahern <dsa@cumulusnetworks.com>