cgroup: rename subsys_bits to subsys_mask

In a previous discussion, Tejun Heo suggested to rename references to
subsys_bits (added_bits, removed_bits, etc) by something more meaningful.

Cc: Li Zefan <lizefan@huawei.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Hillf Danton <dhillf@gmail.com>
Cc: Lennart Poettering <lpoetter@redhat.com>
Signed-off-by: Aristeu Rozanski <aris@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>