NFS: Set key construction data for the legacy upcall

This prevents a null pointer dereference when
nfs_idmap_complete_pipe_upcall_locked() calls complete_request_key().

Fixes a regression caused by commit 0cac1202 (NFSv4: Ensure that
idmap_pipe_downcall sanity-checks the downcall data).
Signed-off-by: Bryan Schumaker <bjschuma@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/idmap.c

@@ -707,6 +707,7 @@ static int nfs_idmap_legacy_upcall(struct key_construction *cons,
 	msg = &data->pipe_msg;
 	im = &data->idmap_msg;
 	data->idmap = idmap;
+	data->key_cons = cons;
 
 	ret = nfs_idmap_prepare_message(key->description, idmap, im, msg);
 	if (ret < 0)