NFS: Fix return type of nfs4_end_drain_session() stub

Clean up: when NFSv4.1 support is compiled out,
nfs4_end_drain_session() becomes a stub.  Make the synopsis of the
stub match the synopsis of the real version of the function.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/nfs4state.c

@@ -2116,7 +2116,7 @@ static int nfs4_bind_conn_to_session(struct nfs_client *clp)
 }
 #else /* CONFIG_NFS_V4_1 */
 static int nfs4_reset_session(struct nfs_client *clp) { return 0; }
-static int nfs4_end_drain_session(struct nfs_client *clp) { return 0; }
+static void nfs4_end_drain_session(struct nfs_client *clp) { }
 
 static int nfs4_bind_conn_to_session(struct nfs_client *clp)
 {