cgroup: cgroup refcnt functions should be exported when CONFIG_DEBUG_CGROUP_REF

6ab428604f72 ("cgroup: Implement DEBUG_CGROUP_REF") added a config option
which forces cgroup refcnt functions to be not inlined so that they can be
kprobed for debugging. However, it forgot export them when the config is
enabled breaking modules which make use of css reference counting.

Fix it by adding CGROUP_REF_EXPORT() macro to cgroup_refcnt.h which is
defined to EXPORT_SYMBOL_GPL when CONFIG_DEBUG_CGROUP_REF is set.

Signed-off-by: Tejun Heo <[e-mail]>
Fixes: 6ab428604f72 ("cgroup: Implement DEBUG_CGROUP_REF")

include/linux/cgroup.h

···
 void css_put_many(struct cgroup_subsys_state *css, unsigned int n);
 #else
 #define CGROUP_REF_FN_ATTRS	static inline
+#define CGROUP_REF_EXPORT(fn)
 #include <linux/cgroup_refcnt.h>
 #endif
 

include/linux/cgroup_refcnt.h

···
 	if (!(css->flags & CSS_NO_REF))
 		percpu_ref_get(&css->refcnt);
 }
+CGROUP_REF_EXPORT(css_get)
 
 /**
  * css_get_many - obtain references on the specified css
···
 	if (!(css->flags & CSS_NO_REF))
 		percpu_ref_get_many(&css->refcnt, n);
 }
+CGROUP_REF_EXPORT(css_get_many)
 
 /**
  * css_tryget - try to obtain a reference on the specified css
···
 		return percpu_ref_tryget(&css->refcnt);
 	return true;
 }
+CGROUP_REF_EXPORT(css_tryget)
 
 /**
  * css_tryget_online - try to obtain a reference on the specified css if online
···
 		return percpu_ref_tryget_live(&css->refcnt);
 	return true;
 }
+CGROUP_REF_EXPORT(css_tryget_online)
 
 /**
  * css_put - put a css reference
···
 	if (!(css->flags & CSS_NO_REF))
 		percpu_ref_put(&css->refcnt);
 }
+CGROUP_REF_EXPORT(css_put)
 
 /**
  * css_put_many - put css references
···
 	if (!(css->flags & CSS_NO_REF))
 		percpu_ref_put_many(&css->refcnt, n);
 }
+CGROUP_REF_EXPORT(css_put_many)

kernel/cgroup/cgroup.c

···
 
 #ifdef CONFIG_DEBUG_CGROUP_REF
 #define CGROUP_REF_FN_ATTRS	noinline
+#define CGROUP_REF_EXPORT(fn)	EXPORT_SYMBOL_GPL(fn);
 #include <linux/cgroup_refcnt.h>
 #endif