writeback: make default number of writeback threads equal to num cpus

This will help test fstests with more number of writeback threads.

Signed-off-by: Kundan Kumar <kundan.kumar@samsung.com>

mm/backing-dev.c

diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index d746ed0..15404fe 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -1185,7 +1185,7 @@ int bdi_init(struct backing_dev_info *bdi)
 	 * introduced sysfs knob.
 	 * echo N > /sys/class/bdi/<maj>:<min>/nwritebacks
 	 */
-	bdi->nr_wb_ctx = 1;
+	bdi->nr_wb_ctx = num_online_cpus();
 
 	bdi->wb_ctx = wb_ctx_alloc(bdi, bdi->nr_wb_ctx);
 	if (!bdi->wb_ctx)