pNFS/flexfiles: Fix layoutstat periodic reporting

Putting the periodicity timer in the mirror instances is causing
non-scalable reporting behaviour and missed reporting intervals.
When you recall layouts and/or implement client side mirroring, it
leads to consecutive reports with only a few ms between RPC calls.

Signed-off-by: Trond Myklebust <[e-mail]>
Fixes: d0379a5d066a9 ("pNFS/flexfiles: Support server-supplied...")

fs/nfs/flexfilelayout/flexfilelayout.c

···
 	if (ffl) {
 		INIT_LIST_HEAD(&ffl->error_list);
 		INIT_LIST_HEAD(&ffl->mirrors);
+		ffl->last_report_time = ktime_get();
 		return &ffl->generic_hdr;
 	} else
 		return NULL;
···
 {
 	static const ktime_t notime = {0};
 	s64 report_interval = FF_LAYOUTSTATS_REPORT_INTERVAL;
+	struct nfs4_flexfile_layout *ffl = FF_LAYOUT_FROM_HDR(mirror->layout);
 
 	nfs4_ff_start_busy_timer(&layoutstat->busy_timer, now);
 	if (ktime_equal(mirror->start_time, notime))
 		mirror->start_time = now;
-	if (ktime_equal(mirror->last_report_time, notime))
-		mirror->last_report_time = now;
 	if (mirror->report_interval != 0)
 		report_interval = (s64)mirror->report_interval * 1000LL;
 	else if (layoutstats_timer != 0)
 		report_interval = (s64)layoutstats_timer * 1000LL;
-	if (ktime_to_ms(ktime_sub(now, mirror->last_report_time)) >=
+	if (ktime_to_ms(ktime_sub(now, ffl->last_report_time)) >=
 			report_interval) {
-		mirror->last_report_time = now;
+		ffl->last_report_time = now;
 		return true;
 	}
 

fs/nfs/flexfilelayout/flexfilelayout.h

···
 	struct nfs4_ff_layoutstat	read_stat;
 	struct nfs4_ff_layoutstat	write_stat;
 	ktime_t				start_time;
-	ktime_t				last_report_time;
 	u32				report_interval;
 };
 
···
 	struct pnfs_ds_commit_info commit_info;
 	struct list_head	mirrors;
 	struct list_head	error_list; /* nfs4_ff_layout_ds_err */
+	ktime_t			last_report_time; /* Layoutstat report times */
 };
 
 static inline struct nfs4_flexfile_layout *