chmod_file: Introduce intelligent file permissions setting

chmod_file: Introduce intelligent file permissions setting

There is no general rule on which permissions make sense for files in
different subsystems. Nevertheless the kernel creates those files with
the maximum allowed permissions for owner so we should use its part as
an umask for group and others permissions as well.
This means that if we specify 777 for file_mode we will end up having
same permissions as owner what ever they are.

The primary intelligence is implemented in cg_chmod_path function which
takes an argument which says whether user permissions should be used
as a template.

This patch adds and exports cgroup_set_permissions which sets
permissions for control and task files into the cgroup descriptor
and cgroup_create_cgroup does the rest.

Example:
/etc/cgconfig.conf:
mount {
        cpu = /cgroup/cpuctl/;
}

group devel {
        perm {
                task {
                        uid = root;
                        gid = cgroup;
                        fperm = 770;
                }
                admin {
                        uid = root;
                        gid = cgroup;
                        dperm = 775;
                        fperm = 770;
                }
        }
        cpu {
                cpu.shares = 5120;
        }
}

cd /cgroup/cpuctl/devel/
ls -la
drwxrwxr-x 2 root cgroup 0 May 19 16:42 .
drwxr-xr-x 4 root root   0 May 19 16:14 ..
-rw-rw---- 1 root cgroup 0 May 19 16:42 cgroup.clone_children
--w--w---- 1 root cgroup 0 May 19 16:42 cgroup.event_control
-r--r----- 1 root cgroup 0 May 19 16:42 cgroup.procs
-rw-rw---- 1 root cgroup 0 May 19 16:42 cpu.rt_period_us
-rw-rw---- 1 root cgroup 0 May 19 16:42 cpu.rt_runtime_us
-rw-rw---- 1 root cgroup 0 May 19 16:42 cpu.shares
-rw-rw---- 1 root cgroup 0 May 19 16:42 notify_on_release
-rw-rw---- 1 root cgroup 0 May 19 16:42 tasks

Signed-off-by: Michal Hocko <mhocko@suse.cz>
Signed-off-by: Jan Safranek <jsafrane@redhat.com>