Canonicalize cgroup paths

Make it possible to verify whether the memory cgroup controller has been
mounted in the v2 hierarchy by comparing its path with the cgroup v2
path. This patch changes the path of cgroup controllers mounted in the
v2 hierarchy from /sys/fs/cgroup/. into /sys/fs/cgroup (no trailing /.).

Bug: 213617178
Test: Added debug code that prints the path for the memory cgroup in the
Test: v2 hierarchy.
Change-Id: I8a2764a6daae84caecf360a918eab62778e3f546
Signed-off-by: Bart Van Assche <bvanassche@google.com>