pNFS/flexfiles: track when layout is first used

So that we can report cumulative time since the beginning
of statistics collection of the layout.

Reviewed-by: Jeff Layton <jeff.layton@primarydata.com>
Signed-off-by: Peng Tao <tao.peng@primarydata.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>

fs/nfs/flexfilelayout/flexfilelayout.h

@@ -74,6 +74,7 @@ struct nfs4_ff_layout_mirror {
 	spinlock_t			lock;
 	struct nfs4_ff_layoutstat	read_stat;
 	struct nfs4_ff_layoutstat	write_stat;
+	ktime_t				start_time;
 };
 
 struct nfs4_ff_layout_segment {