pNFS: LAYOUTRETURN should only update the stateid if the layout is valid

If the layout was completely returned, then ignore the returned layout
stateid.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
diff --git a/fs/nfs/pnfs.h b/fs/nfs/pnfs.h
index b21bd0b..2f4f269 100644
--- a/fs/nfs/pnfs.h
+++ b/fs/nfs/pnfs.h
@@ -375,6 +375,11 @@
 	return NFS_I(inode)->layout != NULL;
 }
 
+static inline bool pnfs_layout_is_valid(const struct pnfs_layout_hdr *lo)
+{
+	return test_bit(NFS_LAYOUT_INVALID_STID, &lo->plh_flags) == 0;
+}
+
 static inline struct nfs4_deviceid_node *
 nfs4_get_deviceid(struct nfs4_deviceid_node *d)
 {