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 <trond.myklebust@primarydata.com>
Fixes: d0379a5d066a9 ("pNFS/flexfiles: Support server-supplied...")
diff --git a/fs/nfs/flexfilelayout/flexfilelayout.h b/fs/nfs/flexfilelayout/flexfilelayout.h
index 1bcdb15..3ee0c9fc 100644
--- a/fs/nfs/flexfilelayout/flexfilelayout.h
+++ b/fs/nfs/flexfilelayout/flexfilelayout.h
@@ -84,7 +84,6 @@
 	struct nfs4_ff_layoutstat	read_stat;
 	struct nfs4_ff_layoutstat	write_stat;
 	ktime_t				start_time;
-	ktime_t				last_report_time;
 	u32				report_interval;
 };
 
@@ -101,6 +100,7 @@
 	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 *