9p/xen: Fix format argument warning

Fix follow warnings:
[net/9p/trans_xen.c:454]: (warning) %u in format string (no. 1) requires
'unsigned int' but the argument type is 'int'.
[net/9p/trans_xen.c:460]: (warning) %u in format string (no. 1) requires
'unsigned int' but the argument type is 'int'.

Link: http://lkml.kernel.org/r/20201009080552.89918-1-yebin10@huawei.comReported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Ye Bin <yebin10@huawei.com>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>

net/9p/trans_xen.c

@@ -451,13 +451,13 @@ static int xen_9pfs_front_probe(struct xenbus_device *dev,
 		char str[16];
 
 		BUILD_BUG_ON(XEN_9PFS_NUM_RINGS > 9);
-		sprintf(str, "ring-ref%u", i);
+		sprintf(str, "ring-ref%d", i);
 		ret = xenbus_printf(xbt, dev->nodename, str, "%d",
 				    priv->rings[i].ref);
 		if (ret)
 			goto error_xenbus;
 
-		sprintf(str, "event-channel-%u", i);
+		sprintf(str, "event-channel-%d", i);
 		ret = xenbus_printf(xbt, dev->nodename, str, "%u",
 				    priv->rings[i].evtchn);
 		if (ret)