cgroup: Wrap subsystem selection macro

Before we are able to define all subsystem ids at compile time we need
a more fine grained control what gets defined when we include
cgroup_subsys.h. For example we define the enums for the subsystems or
to declare for struct cgroup_subsys (builtin subsystem) by including
cgroup_subsys.h and defining SUBSYS accordingly.

Currently, the decision if a subsys is used is defined inside the
header by testing if CONFIG_*=y is true. By moving this test outside
of cgroup_subsys.h we are able to control it on the include level.

This is done by introducing IS_SUBSYS_ENABLED which then is defined
according the task, e.g. is CONFIG_*=y or CONFIG_*=m.

Signed-off-by: Daniel Wagner <[email protected]>
Signed-off-by: Tejun Heo <[email protected]>
Acked-by: Li Zefan <[email protected]>
Acked-by: Neil Horman <[email protected]>
Cc: Gao feng <[email protected]>
Cc: Jamal Hadi Salim <[email protected]>
Cc: John Fastabend <[email protected]>
Cc: [email protected]
Cc: [email protected]

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