pnfs/flexfiles: Add tracepoints for detecting pnfs fallback to MDS

Add tracepoints to allow debugging of the event chain leading to
a pnfs fallback to doing I/O through the MDS.

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

include/linux/nfs4.h

@@ -660,6 +660,7 @@ enum pnfs_update_layout_reason {
 	PNFS_UPDATE_LAYOUT_BLOCKED,
 	PNFS_UPDATE_LAYOUT_INVALID_OPEN,
 	PNFS_UPDATE_LAYOUT_SEND_LAYOUTGET,
+	PNFS_UPDATE_LAYOUT_EXIT,
 };
 
 #define NFS4_OP_MAP_NUM_LONGS					\