um: virtio: fix handling of messages without payload

If we have a message without payload, we call full_read() with
len set to 0, which causes it to return -ECONNRESET. Catch this
case and explicitly return 0 for it so we can actually use the
zero-size config-changed message.
Signed-off-by: NJohannes Berg <johannes.berg@intel.com>
Signed-off-by: NRichard Weinberger <richard@nod.at>

arch/um/drivers/virtio_uml.c

@@ -97,6 +97,9 @@ static int full_read(int fd, void *buf, int len, bool abortable)
 {
 	int rc;
 
+	if (!len)
+		return 0;
+
 	do {
 		rc = os_read_file(fd, buf, len);
 		if (rc > 0) {