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>
diff --git a/fs/nfs/pnfs.c b/fs/nfs/pnfs.c
index 7589174..75bd5b5 100644
--- a/fs/nfs/pnfs.c
+++ b/fs/nfs/pnfs.c
@@ -2037,6 +2037,8 @@ pnfs_update_layout(struct inode *ino,
 out_put_layout_hdr:
 	if (first)
 		pnfs_clear_first_layoutget(lo);
+	trace_pnfs_update_layout(ino, pos, count, iomode, lo, lseg,
+				 PNFS_UPDATE_LAYOUT_EXIT);
 	pnfs_put_layout_hdr(lo);
 out:
 	dprintk("%s: inode %s/%llu pNFS layout segment %s for "