mptcp: initialize mptcp_options_received's ahmac

Initialize mptcp_options_received's ahmac to zero, otherwise it
will be a random number when receiving ADD_ADDR suboption with echo-flag=1.

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: Jakub Kicinski <kuba@kernel.org>

net/mptcp/options.c

@@ -297,6 +297,7 @@ void mptcp_get_options(const struct sk_buff *skb,
 	mp_opt->mp_capable = 0;
 	mp_opt->mp_join = 0;
 	mp_opt->add_addr = 0;
+	mp_opt->ahmac = 0;
 	mp_opt->rm_addr = 0;
 	mp_opt->dss = 0;