cgroupv2/systemd: add getv2Path
cgroupv2/systemd: add getv2Path

Function getSubsystemPath(), while works for v2 unified case, is
suboptimal, as it does a few unnecessary calls.

Add a simplified version of getSubsystemPath(), called getv2Path(),
and use it.

Signed-off-by: Kir Kolyshkin <3a017b8ddb3f9cf3e4a59978b004111bdeb97f08@gmail.com>

mikebrow/runc,rhatdan/runc,kolyshkin/runc,estesp/runc,mikebrow/runc,kolyshkin/runc,xiaochenshen/runc,opencontainers/runc,rhatdan/runc,dqminh/runc,rhatdan/runc,opencontainers/runc,opencontainers/runc,kolyshkin/runc,xiaochenshen/runc,estesp/runc,estesp/runc,dqminh/runc,mikebrow/runc,dqminh/runc,rhatdan/runc,opencontainers/runc,kolyshkin/runc,xiaochenshen/runc,dqminh/runc