kthread: unexport kthread_blkcg

kthread_blkcg is only used by the built-in blk-cgroup code.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20220420042723.1010598-16-hch@lst.deSigned-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/kthread.h

@@ -222,9 +222,5 @@ void kthread_associate_blkcg(struct cgroup_subsys_state *css);
 struct cgroup_subsys_state *kthread_blkcg(void);
 #else
 static inline void kthread_associate_blkcg(struct cgroup_subsys_state *css) { }
-static inline struct cgroup_subsys_state *kthread_blkcg(void)
-{
-	return NULL;
-}
 #endif
 #endif /* _LINUX_KTHREAD_H */

kernel/kthread.c

@@ -1522,5 +1522,4 @@ struct cgroup_subsys_state *kthread_blkcg(void)
 	}
 	return NULL;
 }
-EXPORT_SYMBOL(kthread_blkcg);
 #endif