writeback: add @gfp to wb_init()

wb_init() currently always uses GFP_KERNEL but the planned cgroup
writeback support needs using other allocation masks.  Add @gfp to
wb_init().

This patch doesn't introduce any behavior changes.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Jens Axboe <axboe@fb.com>

mm/backing-dev.c

@@ -291,7 +291,8 @@ void wb_wakeup_delayed(struct bdi_writeback *wb)
  */
 #define INIT_BW		(100 << (20 - PAGE_SHIFT))
 
-static int wb_init(struct bdi_writeback *wb, struct backing_dev_info *bdi)
+static int wb_init(struct bdi_writeback *wb, struct backing_dev_info *bdi,
+		   gfp_t gfp)
 {
 	int i, err;
 
@@ -315,12 +316,12 @@ static int wb_init(struct bdi_writeback *wb, struct backing_dev_info *bdi)
 	INIT_LIST_HEAD(&wb->work_list);
 	INIT_DELAYED_WORK(&wb->dwork, wb_workfn);
 
-	err = fprop_local_init_percpu(&wb->completions, GFP_KERNEL);
+	err = fprop_local_init_percpu(&wb->completions, gfp);
 	if (err)
 		return err;
 
 	for (i = 0; i < NR_WB_STAT_ITEMS; i++) {
-		err = percpu_counter_init(&wb->stat[i], 0, GFP_KERNEL);
+		err = percpu_counter_init(&wb->stat[i], 0, gfp);
 		if (err) {
 			while (--i)
 				percpu_counter_destroy(&wb->stat[i]);
@@ -378,7 +379,7 @@ int bdi_init(struct backing_dev_info *bdi)
 	bdi->max_prop_frac = FPROP_FRAC_BASE;
 	INIT_LIST_HEAD(&bdi->bdi_list);
 
-	err = wb_init(&bdi->wb, bdi);
+	err = wb_init(&bdi->wb, bdi, GFP_KERNEL);
 	if (err)
 		return err;