QIOChannelSocket: Fix zero-copy flush returning code 1 when nothing sent

If flush is called when no buffer was sent with MSG_ZEROCOPY, it currently
returns 1. This return code should be used only when Linux fails to use
MSG_ZEROCOPY on a lot of sendmsg().

Fix this by returning early from flush if no sendmsg(...,MSG_ZEROCOPY)
was attempted.

Fixes: 2bc58ffc ("QIOChannelSocket: Implement io_writev zero copy flag & io_flush for CONFIG_LINUX")
Signed-off-by: NLeonardo Bras <leobras@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>
Acked-by: NDaniel P. Berrangé <berrange@redhat.com>
Reviewed-by: NJuan Quintela <quintela@redhat.com>
Reviewed-by: NPeter Xu <peterx@redhat.com>
Message-Id: <20220711211112.18951-2-leobras@redhat.com>
Signed-off-by: NDr. David Alan Gilbert <dgilbert@redhat.com>

io/channel-socket.c

@@ -716,12 +716,18 @@ static int qio_channel_socket_flush(QIOChannel *ioc,
     struct cmsghdr *cm;
     char control[CMSG_SPACE(sizeof(*serr))];
     int received;
-    int ret = 1;
+    int ret;
+
+    if (sioc->zero_copy_queued == sioc->zero_copy_sent) {
+        return 0;
+    }
 
     msg.msg_control = control;
     msg.msg_controllen = sizeof(control);
     memset(control, 0, sizeof(control));
 
+    ret = 1;
+
     while (sioc->zero_copy_sent < sioc->zero_copy_queued) {
         received = recvmsg(sioc->fd, &msg, MSG_ERRQUEUE);
         if (received < 0) {