ip vrf: Handle vrf in a cgroup hierarchy

ip vrf: Handle vrf in a cgroup hierarchy

Add support for VRF in a pre-existing hierarchy. For example, if the
current process is running in CGRP/foo/bar, the 'ip vrf exec NAME CMD'
should run CMD in the cgroup CGRP/foo/bar/vrf/NAME.

When listing process ids in a VRF, search for the directory vrf/NAME
regardless of base path (foo/bar/vrf/NAME and vrf/NAME) are still
running against the same vrf NAME.

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