NFS: Cleanup - only store the write verifier in struct nfs_page

The 'committed' field is not needed once we have put the struct nfs_page
on the right list.

Also correct the type of the verifier: it is not an array of __be32, but
simply an 8 byte long opaque array.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

include/linux/nfs_xdr.h

@@ -514,9 +514,13 @@ struct nfs_writeargs {
 	struct nfs4_sequence_args	seq_args;
 };
 
+struct nfs_write_verifier {
+	char			data[8];
+};
+
 struct nfs_writeverf {
+	struct nfs_write_verifier verifier;
 	enum nfs3_stable_how	committed;
-	__be32			verifier[2];
 };
 
 struct nfs_writeres {