Show virtualized dentry root in mountinfo for cgroupfs

[serge . hallyn]

With the current cgroup namespace patches, the root dentry path of a
mount as shown in /proc/self/mountinfo is the full global cgroup
path.  It is common for userspace to use /proc/self/mountinfo to
search for cgroup mountpoints, and expect the root dentry path to
relate to the cgroup paths in /proc/self/cgroup.  Patch 2 in this
set therefore virtualizes the root dentry path relative to the
reader's cgroup namespace root.

Patch 1 fixes a bug in kernfs_path_from_node_locked() which is
exposed by patch 2.