mptcp: print new line in mptcp_seq_show() if mptcp isn't in use

When do cat /proc/net/netstat, the output isn't append with a new line, it looks like this:
[root@localhost ~]# cat /proc/net/netstat
...
MPTcpExt: 0 0 0 0 0 0 0 0 0 0 0 0 0[root@localhost ~]#

This is because in mptcp_seq_show(), if mptcp isn't in use, net->mib.mptcp_statistics is NULL,
so it just puts all 0 after "MPTcpExt:", and return, forgot the '\n'.

After this patch:

[root@localhost ~]# cat /proc/net/netstat
...
MPTcpExt: 0 0 0 0 0 0 0 0 0 0 0 0 0
[root@localhost ~]#

Fixes: fc518953bc ("mptcp: add and use MIB counter infrastructure")
Signed-off-by: Jianguo Wu <wujianguo@chinatelecom.cn>
Acked-by: Florian Westphal <fw@strlen.de>
Link: https://lore.kernel.org/r/142e2fd9-58d9-bb13-fb75-951cccc2331e@163.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/mptcp/mib.c

@@ -67,6 +67,7 @@ void mptcp_seq_show(struct seq_file *seq)
 		for (i = 0; mptcp_snmp_list[i].name; i++)
 			seq_puts(seq, " 0");
 
+		seq_putc(seq, '\n');
 		return;
 	}