cgroup: use bitmask to filter for_each_subsys

Add a new macro for_each_subsys_which that allows all enabled cgroup
subsystems to be filtered by a bitmask, such that mask & (1 << ssid)
determines if the subsystem is to be processed in the loop body (where
ssid is the unique id of the subsystem).

Also replace the need_forkexit_callback with two separate bitmasks for
each callback to make (ss->{fork,exit}) checks unnecessary.

tj: add a short comment for "if (!CGROUP_SUBSYS_COUNT)".

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

include/linux/cgroup-defs.h

@@ -490,6 +490,8 @@ static inline void cgroup_threadgroup_change_end(struct task_struct *tsk)
 
 #else	/* CONFIG_CGROUPS */
 
+#define CGROUP_SUBSYS_COUNT 0
+
 static inline void cgroup_threadgroup_change_begin(struct task_struct *tsk) {}
 static inline void cgroup_threadgroup_change_end(struct task_struct *tsk) {}