writeback: initialize global_dirty_limit

This prevents global_dirty_limit from remaining 0 (the initial value)
for long time, since it's only updated in update_dirty_limit() when
above the dirty freerun area.

It will avoid unexpected consequences when some random code use it as a
convenient approximation of the global dirty threshold.
Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>

mm/page-writeback.c

@@ -1568,6 +1568,7 @@ void writeback_set_ratelimit(void)
 	unsigned long background_thresh;
 	unsigned long dirty_thresh;
 	global_dirty_limits(&background_thresh, &dirty_thresh);
+	global_dirty_limit = dirty_thresh;
 	ratelimit_pages = dirty_thresh / (num_online_cpus() * 32);
 	if (ratelimit_pages < 16)
 		ratelimit_pages = 16;