mptcp: fix ADD_ADDR HMAC in case port is specified

Currently, Linux computes the HMAC contained in ADD_ADDR sub-option using
the Address Id and the IP Address, and hardcodes a destination port equal
to zero. This is not ok for ADD_ADDR with port: ensure to account for the
endpoint port when computing the HMAC, in compliance with RFC8684 §3.4.1.

Fixes: 22fb85ff ("mptcp: add port support for ADD_ADDR suboption writing")
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Acked-by: Geliang Tang <geliangtang@gmail.com>
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>