NFS: Fix a compile issue when CONFIG_NFS_V4_1 is undefined

struct nfs_direct_req can't compile when struct pnfs_ds_commit_info
is undefined.
Reported-by: NBryan Schumaker <bjschuma@netapp.com>
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>
Cc: Fred Isaman <iisaman@netapp.com>

include/linux/nfs_xdr.h

@@ -1176,6 +1176,11 @@ struct nfs41_free_stateid_res {
 	struct nfs4_sequence_res	seq_res;
 };
 
+#else
+
+struct pnfs_ds_commit_info {
+};
+
 #endif /* CONFIG_NFS_V4_1 */
 
 struct nfs_page;