mptcp: bugfix for RM_ADDR option parsing

In MPTCPOPT_RM_ADDR option parsing, the pointer "ptr" pointed to the
"Subtype" octet, the pointer "ptr+1" pointed to the "Address ID" octet:

  +-------+-------+---------------+
  |Subtype|(resvd)|   Address ID  |
  +-------+-------+---------------+
  |               |
 ptr            ptr+1

We should set mp_opt->rm_id to the value of "ptr+1", not "ptr". This patch
will fix this bug.

Fixes: 3df523ab ("mptcp: Add ADD_ADDR handling")
Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mptcp/options.c

@@ -273,6 +273,8 @@ static void mptcp_parse_option(const struct sk_buff *skb,
 		if (opsize != TCPOLEN_MPTCP_RM_ADDR_BASE)
 			break;
 
+		ptr++;
+
 		mp_opt->rm_addr = 1;
 		mp_opt->rm_id = *ptr++;
 		pr_debug("RM_ADDR: id=%d", mp_opt->rm_id);