mptcp: make sure flag signal is set when add addr with port

When add address with port, it is mean to create a listening socket,
and send an ADD_ADDR to remote, so it must have flag signal set,
add this check in mptcp_pm_parse_addr().

Fixes: a77e9179 ("mptcp: deal with MPTCP_PM_ADDR_ATTR_PORT in PM netlink")
Acked-by: Geliang Tang <geliangtang@gmail.com>
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jianguo Wu <wujianguo@chinatelecom.cn>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>