mptcp: Fix union in tcp_out_options

It may be that a MP_CAPABLE is combined with an ADD_ADDR if there is
enough option-space. Then, the address will overwrite the key in the
MP_CAPABLE because it's within the same union in tcp_out_options.

We thus cannot use a union here.

Reported-by: Krishna Khanal <erkrishna.khanal@gmail.com>
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>