UPSTREAM: cgroup, blkcg: Prepare some symbols for module and !CONFIG_CGROUP usages

btrfs is going to use css_put() and wbc helpers to improve cgroup
writeback support.  Add dummy css_get() definition and export wbc
helpers to prepare for module and !CONFIG_CGROUP builds.

Reported-by: kbuild test robot <lkp@intel.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Change-Id: I2fef472a0ce04d6bb32213500f57c72045b13633
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-cgroup.c

@@ -46,6 +46,7 @@ struct blkcg blkcg_root;
 EXPORT_SYMBOL_GPL(blkcg_root);
 
 struct cgroup_subsys_state * const blkcg_root_css = &blkcg_root.css;
+EXPORT_SYMBOL_GPL(blkcg_root_css);
 
 static struct blkcg_policy *blkcg_policy[BLKCG_MAX_POLS];
 

fs/fs-writeback.c

@@ -598,6 +598,7 @@ void wbc_attach_and_unlock_inode(struct writeback_control *wbc,
 	if (unlikely(wb_dying(wbc->wb) && !css_is_dying(wbc->wb->memcg_css)))
 		inode_switch_wbs(inode, wbc->wb_id);
 }
+EXPORT_SYMBOL_GPL(wbc_attach_and_unlock_inode);
 
 /**
  * wbc_detach_inode - disassociate wbc from inode and perform foreign detection
@@ -717,6 +718,7 @@ void wbc_detach_inode(struct writeback_control *wbc)
 	wb_put(wbc->wb);
 	wbc->wb = NULL;
 }
+EXPORT_SYMBOL_GPL(wbc_detach_inode);
 
 /**
  * wbc_account_io - account IO issued during writeback

include/linux/cgroup.h

@@ -700,6 +700,7 @@ void cgroup_path_from_kernfs_id(u64 id, char *buf, size_t buflen);
 struct cgroup_subsys_state;
 struct cgroup;
 
+static inline void css_get(struct cgroup_subsys_state *css) {}
 static inline void css_put(struct cgroup_subsys_state *css) {}
 static inline int cgroup_attach_task_all(struct task_struct *from,
 					 struct task_struct *t) { return 0; }