We should join the cgroup after the initial setup finished,
but before runc init clone new children processes. (opencontainers#4427)
Because we should try our best to reduce the influence of
memory cgroup accounting from all runc init processes
before we start the container init process.

Signed-off-by: lifubang <lifubang@acmcoder.com>

As we will fix the race between binary clone and cgroup join, we can eliminate the
impacts of memory accounting from ensure_clone_binary. So runc will support lower memory
useage the same as before.

This reverts commit 719e2bc.

Signed-off-by: lifubang <lifubang@acmcoder.com>