mm/page-writeback.c: Fix comment of wb_domain_writeout_add()

__bdi_writeout_inc() has undergone multiple renamings, but the comment
within the function body have not been updated accordingly. Update it
to reflect the latest wb_domain_writeout_add().

Signed-off-by: Tang Yizhou <yizhou.tang@shopee.com>
Link: https://lore.kernel.org/r/20241009151728.300477-3-yizhou.tang@shopee.com
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: Tang Yizhou

mm/page-writeback.c

@@ -586,7 +586,7 @@ static void wb_domain_writeout_add(struct wb_domain *dom,
 	/* First event after period switching was turned off? */
 	if (unlikely(!dom->period_time)) {
 		/*
-		 * We can race with other __bdi_writeout_inc calls here but
+		 * We can race with other wb_domain_writeout_add calls here but
 		 * it does not cause any harm since the resulting time when
 		 * timer will fire and what is in writeout_period_time will be
 		 * roughly the same.