libstand: nfs_readlink() should return proper return code

The nfs_readlink() is returning constant 0 instead of variable.

Reviewed by:	avg
Differential Revision:	https://reviews.freebsd.org/D12201

lib/libstand/nfs.c

@@ -382,7 +382,7 @@ nfs_readlink(struct nfs_iodesc *d, char *buf)
 	buf[repl->len] = 0;
 done:
 	free(pkt);
-	return (0);
+	return (rc);
 }
 #endif