cg: Trim proper prefix from ftw's path

After the commit that walks /proc/self/fd/N path instead of the temporary
one, the add_cgroup() started trimming first several bytes from the cgroup
path.

Test passed, since all cgroups were left as is after dump, so criu restore
didn't recreate them but got EEXIST on all mkdir-s.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Acked-by: Tycho Andersen <tycho.andersen@canonical.com>
cg: Keep "/" in cgroup root paths

The ftw trims tail "/" from path argument.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Acked-by: Tycho Andersen <tycho.andersen@canonical.com>
cg: Merge two equal if-s into one

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Acked-by: Tycho Andersen <tycho.andersen@canonical.com>