mptcp: unify add_addr(6)_generate_hmac

The length of the IPv4 address is 4 octets and IPv6 is 16. That's the only
difference between add_addr_generate_hmac and add_addr6_generate_hmac.

This patch dropped the duplicate code and unify them into one.

Co-developed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>