writeback: make wb_start_writeback() static

We don't have any callers outside of fs-writeback.c anymore,
make it private.

Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Tested-by: Chris Mason <clm@fb.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/fs-writeback.c

@@ -933,8 +933,8 @@ static void bdi_split_work_to_wbs(struct backing_dev_info *bdi,
 
 #endif	/* CONFIG_CGROUP_WRITEBACK */
 
-void wb_start_writeback(struct bdi_writeback *wb, long nr_pages,
+static void wb_start_writeback(struct bdi_writeback *wb, long nr_pages,
-			enum wb_reason reason)
+				enum wb_reason reason)
 {
 	struct wb_writeback_work *work;
 

include/linux/backing-dev.h

@@ -38,8 +38,6 @@ static inline struct backing_dev_info *bdi_alloc(gfp_t gfp_mask)
 	return bdi_alloc_node(gfp_mask, NUMA_NO_NODE);
 }
 
-void wb_start_writeback(struct bdi_writeback *wb, long nr_pages,
-			enum wb_reason reason);
 void wb_start_background_writeback(struct bdi_writeback *wb);
 void wb_workfn(struct work_struct *work);
 void wb_wakeup_delayed(struct bdi_writeback *wb);