NFSv4.2/pnfs: Make the layoutstats timer configurable

Allow advanced users to set the layoutstats timer in order to lengthen
or shorten the period between layoutstat transmissions to the server.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>

fs/nfs/pnfs.c

@@ -2285,3 +2285,7 @@ out_put:
 }
 EXPORT_SYMBOL_GPL(pnfs_report_layoutstat);
 #endif
+
+unsigned int layoutstats_timer;
+module_param(layoutstats_timer, uint, 0644);
+EXPORT_SYMBOL_GPL(layoutstats_timer);