Cache cgroup root mount location.
Cache cgroup root mount location.

We calculate this on every cgroup call. It comprised of 30%+ of the CPU
usage in cAdvisor.

Docker-DCO-1.1-Signed-off-by: Victor Marmol <vmarmol@google.com> (github: vmarmol)

boucher/runc,zenlinTechnofreak/runc,mheon/libcontainer,gwoo/runc,zhangyancoder/libcontainer,lizf-os/runc,cyphar/runc,estesp/runc,mheon/runc,han-wang/runc,mikebrow/runc,rhatdan/runc,clnperez/runc,shakamunyi/libcontainer,gitido/runc,kolyshkin/runc,jwuwork/runc,hqhq/runc,nkwilson/runc,ekuric/runc,dieface/runc,inatatsu/runc,hqhq/libcontainer,wking/runc,kolyshkin/runc,avagin/libcontainer,markllama/runc,jhowardmsft/runc,navidshaikh/runc,zhangyancoder/libcontainer,coolljt0725/runc,mapk0y/runc,LTD-Beget/libcontainer,keloyang/libcontainer,estesp/runc,ZJU-SEL/libcontainer,shishir-a412ed/runc,keloyang/libcontainer,rootfs/runc,inatatsu/runc,ChengTiesheng/runc,rhatdan/runc,LTD-Beget/libcontainer,keloyang/libcontainer,SaiedKazemi/libcontainer-criu,inatatsu/runc,hqhq/runc,justincormack/runc,subuser-security/runc,carlye566/runc,ycaihua/libcontainer,estesp/libcontainer,HardikDR/runc,LTD-Beget/libcontainer,YujiOshima/runc,dqminh/runc,kolyshkin/runc,chenchun/runc,Mashimiao/libcontainer,mYmNeo/runc,w4ngyi/libcontainer,mheon/runc,cxxly/runc,xiaochenshen/runc,coolljt0725/runc,laijs/runc,maxamillion/runc,icecrime/libcontainer,fabiokung/runc,w4ngyi/libcontainer,shakamunyi/libcontainer,mapk0y/runc,ChengTiesheng/runc,haijunTan/runc,avagin/runc,thaJeztah/libcontainer,InformatiQ/runc,boucher/libcontainer,talwai/runc,opencontainers/runc,adrianreber/runc,shenxiaochen/libcontainer,crosbymichael/libcontainer,balajibr/runc,dqminh/libcontainer,gitido/runc,tangkun75/libcontainer,navidshaikh/runc,zhangyancoder/libcontainer,axsh/runc,booyaa/runc,clnperez/runc,xiaochenshen/runc,unclejack/libcontainer,ycaihua/runc,cloudfoundry-incubator/runc,HardikDR/runc,avagin/libcontainer,intfrr/runc,dieface/runc,mikebrow/runc,han-wang/runc,BrickXu/libcontainer,mheon/libcontainer,albertoleal/runc,liuwharton/runc,hxfeng/runc,chenchun/runc,adrianreber/runc,ycaihua/libcontainer,LK4D4/runc,liuwharton/runc,HuKeping/runc,aidanhs/runc,stefanberger/runc,dqminh/runc,rhatdan/libcontainer,opencontainers/runc,keloyang/libcontainer,avagin/runc,subuser-security/runc,laijs/runc,wcwxyz/runc,dqminh/libcontainer,sallyom/runc,SaiedKazemi/libcontainer-criu,albertoleal/runc,shenxiaochen/libcontainer,wcwxyz/runc,gitido/runc,balajibr/runc,ekuric/runc,willmtemple/runc,abduld/runc,navidshaikh/runc,wking/runc,tangkun75/libcontainer,keloyang/runc,icecrime/libcontainer,adrianreber/runc,mrunalp/libcontainer,docker/libcontainer,jhjeong-kr/runc,cloudfoundry-incubator/runc,Mashimiao/libcontainer,albertoleal/runc,HuKeping/runc,marcosnils/runc,ZJU-SEL/runc,rsampaio/runc,ycaihua/runc,kolyshkin/runc,whuwxl/libcontainer,xiaochenshen/runc,booyaa/runc,marcosnils/runc,fabiokung/runc,dqminh/runc,runcom/libcontainer,InformatiQ/runc,ZJU-SEL/runc,mheon/runc,opencontainers/runc,sunyuan3/runc,crosbymichael/runc,maxamillion/runc,SaiedKazemi/libcontainer-criu,estesp/runc,estesp/libcontainer,rhatdan/runc,vbatts/runc,intfrr/runc,mYmNeo/runc,unclejack/libcontainer,wking/runc,zenlinTechnofreak/runc,rootfs/runc,wangkirin/runc,kimh/runc,mrunalp/runc,runcom/runc,mrunalp/runc,tangkun75/libcontainer,vbatts/runc,coolljt0725/runc,abduld/runc,HardikDR/runc,rhatdan/runc,Microsoft/libcontainer,opencontainers/runc,mheon/libcontainer,BrickXu/libcontainer,docker/runc,meihuabo/runc,shishir-a412ed/runc,markllama/runc,mrunalp/libcontainer,carlye566/runc,balajibr/runc,calavera/runc,ycaihua/runc,chenchun/runc,justincormack/runc,aidanhs/runc,YujiOshima/runc,ChengTiesheng/runc,kimh/runc,avagin/runc,LK4D4/runc,avagin/libcontainer,ycaihua/libcontainer,carlye566/runc,jhowardmsft/runc,runcom/libcontainer,kimh/runc,hqhq/libcontainer,wcwxyz/runc,meihuabo/runc,rajasec/runc,mYmNeo/runc,wangkirin/runc,laijs/runc,zenlinTechnofreak/runc,WeiZhang555/runc,w4ngyi/libcontainer,cloudfoundry-incubator/runc,rajasec/runc,stefanberger/runc,icecrime/libcontainer,Mashimiao/libcontainer,maxamillion/runc,boucher/libcontainer,dieface/runc,willmtemple/runc,mapk0y/runc,aidanhs/runc,intfrr/runc,docker/libcontainer,dqminh/libcontainer,crosbymichael/libcontainer,mrunalp/runc,subuser-security/runc,ddnbgroup/runc,talwai/runc,calavera/runc,whuwxl/libcontainer,mikebrow/runc,sallyom/runc,keloyang/runc,rsampaio/runc,Microsoft/libcontainer,whosthatknocking/runc,jwuwork/runc,abduld/runc,dqminh/runc,ZJU-SEL/libcontainer,rhatdan/libcontainer,willmtemple/runc,ddnbgroup/runc,rootfs/runc,hqhq/runc,unclejack/libcontainer,meihuabo/runc,marcosnils/runc,rajasec/runc,cxxly/runc,runcom/runc,keloyang/runc,crosbymichael/libcontainer,whosthatknocking/runc,ZJU-SEL/libcontainer,Microsoft/libcontainer,WeiZhang555/runc,ddnbgroup/runc,rhatdan/libcontainer,shakamunyi/libcontainer,rsampaio/runc,jhowardmsft/runc,thaJeztah/libcontainer,WeiZhang555/runc,sallyom/runc,talwai/runc,docker/libcontainer,shenxiaochen/libcontainer,LK4D4/runc,lizf-os/runc,axsh/runc,InformatiQ/runc,whuwxl/libcontainer,cyphar/runc,hqhq/libcontainer,hxfeng/runc,boucher/runc,cxxly/runc,wangkirin/runc,calavera/runc,docker/runc,sunyuan3/runc,thaJeztah/libcontainer,hxfeng/runc,lizf-os/runc,sunyuan3/runc,han-wang/runc,YujiOshima/runc,whosthatknocking/runc,booyaa/runc,justincormack/runc,cyphar/runc,haijunTan/runc,stefanberger/runc,mrunalp/libcontainer,crosbymichael/runc,nkwilson/runc,axsh/runc,liuwharton/runc,boucher/runc,runcom/runc,nkwilson/runc,docker/runc,fabiokung/runc,haijunTan/runc,jwuwork/runc,jhjeong-kr/runc,crosbymichael/runc,runcom/libcontainer,BrickXu/libcontainer,clnperez/runc,boucher/libcontainer,vbatts/runc,estesp/libcontainer,markllama/runc,ZJU-SEL/runc,ekuric/runc,HuKeping/runc,shishir-a412ed/runc,jhjeong-kr/runc