cgroup: support mount of cgroup2
cgroup: support mount of cgroup2

convert a "cgroup" mount to "cgroup2" when the system uses cgroups v2
unified hierarchy.

Signed-off-by: Giuseppe Scrivano <20002b7b24c75512093a71a58285826d5c504fca@redhat.com>

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