merge branch 'pr-1500'
merge branch 'pr-1500'

  rootfs: switch ms_private remount of oldroot to ms_slave

LGTMs: 2665d0b6daa6ccd1337288f8885cbc52b5d5ee7e@hqhq
Closes opencontainers/runc#1500

mheon/runc,justincormack/runc,adrianreber/runc,cyphar/runc,rajasec/runc,kolyshkin/runc,estesp/runc,rhatdan/runc,stefanberger/runc,cyphar/runc,laijs/runc,xiaochenshen/runc,wking/runc,opencontainers/runc,stefanberger/runc,kolyshkin/runc,cyphar/runc,sunyuan3/runc,xiaochenshen/runc,rajasec/runc,rhatdan/runc,estesp/runc,mheon/runc,stefanberger/runc,opencontainers/runc,dqminh/runc,laijs/runc,mheon/runc,dqminh/runc,opencontainers/runc,justincormack/runc,dqminh/runc,adrianreber/runc,kolyshkin/runc,xiaochenshen/runc,mikebrow/runc,crosbymichael/runc,mYmNeo/runc,crosbymichael/runc,mYmNeo/runc,kolyshkin/runc,dqminh/runc,mikebrow/runc,opencontainers/runc,wking/runc,estesp/runc,sunyuan3/runc,justincormack/runc,rajasec/runc,laijs/runc,rhatdan/runc,wking/runc,sunyuan3/runc,crosbymichael/runc,rhatdan/runc,mikebrow/runc,adrianreber/runc,mYmNeo/runc