mptcp: fix uninit-value in mptcp_incoming_options

Added initialization use_ack to mptcp_parse_option().

Reported-by: syzbot+b834a6b2decad004cfa1@syzkaller.appspotmail.com
Signed-off-by: Edward Adam Davis <eadavis@qq.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mptcp/options.c

@@ -108,6 +108,7 @@ static void mptcp_parse_option(const struct sk_buff *skb,
 			mp_opt->suboptions |= OPTION_MPTCP_DSS;
 			mp_opt->use_map = 1;
 			mp_opt->mpc_map = 1;
+			mp_opt->use_ack = 0;
 			mp_opt->data_len = get_unaligned_be16(ptr);
 			ptr += 2;
 		}