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>