cr: prepare a container root
cr: prepare a container root

CRIU has following requirements for a container root:
* it is a mount point
* a parent mount point isn't over-mounted

Signed-off-by: Andrey Vagin <74c05afa918858348d6db8037d82fa8124870113@openvz.org>

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