journal-gatewayd: fix tmpfile logic

"rw" is not a valid mode string for f*open(). This got broken in
cc02a7b330 ("journal-gatewayd: factor out opening of temp
file").

src/journal-remote/journal-gatewayd.c

@@ -132,7 +132,7 @@ static int request_meta_ensure_tmp(RequestMeta *m) {
                 if (fd < 0)
                         return fd;
 
-                m->tmp = fdopen(fd, "rw");
+                m->tmp = fdopen(fd, "w+");
                 if (!m->tmp) {
                         safe_close(fd);
                         return -errno;