Add a call to nfscl_mustflush() in nfs_close() of the experimental

NFSv4 client, so that attributes are not acquired from the server
when a delegation for the file is held. This can reduce the number
of Getattr Ops significantly.

MFC after:	2 weeks

sys/fs/nfsclient/nfs_clvnops.c

@@ -709,7 +709,7 @@ nfs_close(struct vop_close_args *ap)
 		/*
 		 * Get attributes so "change" is up to date.
 		 */
-		if (!error) {
+		if (error == 0 && nfscl_mustflush(vp)) {
 			ret = nfsrpc_getattr(vp, cred, ap->a_td, &nfsva,
 			    NULL);
 			if (!ret) {