writeback: add dirty_throttle_control->wb_completions

wb->completions measures the wb's proportional write bandwidth in
global_wb_domain and thus naturally tied to the wb_domain.  This patch
adds dirty_throttle_control->wb_completions which is initialized to
wb->completions by GDTC_INIT() and updates __wb_dirty_limits() to use
it instead of dereferencing wb->completions directly.

This will allow dirty_throttle_control to represent different
wb_domains and the matching wb completions.

This patch doesn't introduce any behavioral changes.
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Jan Kara <jack@suse.cz>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Cc: Greg Thelen <gthelen@google.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

mm/page-writeback.c

@@ -127,6 +127,7 @@ struct wb_domain global_wb_domain;
 /* consolidated parameters for balance_dirty_pages() and its subroutines */
 struct dirty_throttle_control {
 	struct bdi_writeback	*wb;
+	struct fprop_local_percpu *wb_completions;
 
 	unsigned long		dirty;		/* file_dirty + write + nfs */
 	unsigned long		thresh;		/* dirty threshold */
@@ -139,7 +140,8 @@ struct dirty_throttle_control {
 	unsigned long		pos_ratio;
 };
 
-#define GDTC_INIT(__wb)		.wb = (__wb)
+#define GDTC_INIT(__wb)		.wb = (__wb),				\
+				.wb_completions = &(__wb)->completions
 
 /*
  * Length of period for aging writeout fractions of bdis. This is an
@@ -590,7 +592,7 @@ static unsigned long __wb_calc_thresh(struct dirty_throttle_control *dtc)
 	/*
 	 * Calculate this BDI's share of the thresh ratio.
 	 */
-	fprop_fraction_percpu(&dom->completions, &dtc->wb->completions,
+	fprop_fraction_percpu(&dom->completions, dtc->wb_completions,
 			      &numerator, &denominator);
 
 	wb_thresh = (thresh * (100 - bdi_min_ratio)) / 100;